Created Custom DataGrids (markdown)

Custom-DataGrids.md

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+The default `DenseInstances` type may not meet your application's needs. Fortunately, the `DenseInstances` type implements a `FixedDataGrid`, meaning it's easy to adapt for your own needs. 
+
+## Interfaces
+Here's the `DataGrid` for in golearn 0.1:
+```go
+type DataGrid interface {
+    // Retrieves a given Attribute's specification
+    GetAttribute(Attribute) (AttributeSpec, error)
+    // Retrieves details of every Attribute
+    AllAttributes() []Attribute
+    // Marks an Attribute as a class Attribute
+    AddClassAttribute(Attribute) error
+    // Unmarks an Attribute as a class Attribute
+    RemoveClassAttribute(Attribute) error
+    // Returns details of all class Attributes
+    AllClassAttributes() []Attribute
+    // Gets the bytes at a given position or nil
+    Get(AttributeSpec, int) []byte
+    // Convenience function for iteration.
+    MapOverRows([]AttributeSpec, func([][]byte, int) (bool, error)) error
+}
+```
+
+`FixedDataGrid` adds a few extra methods.
+```go
+type FixedDataGrid interface {
+    DataGrid
+    // Returns a string representation of a given row
+    RowString(int) string
+    // Returns the number of Attributes and rows currently allocated
+    Size() (int, int)
+}
+```
+
+[Refer to the automatically up-to-date documentation for more recent versions of GoLearn.](https://godoc.org/github.com/sjwhitworth/golearn/base#DataGrid)
+
+## Functional description
+
+### `GetAttribute` 
+`Attribute` implementations in GoLearn describe features of the machine learning problem. As of GoLearn 0.1, the implementations that exist as part of base are `CategoricalAttribute` and `FloatAttributes` (both 64-bits), as well as `BinaryAttribute`. `AttributeSpec` structures link an `Attribute` to a implementation-specific idea of where the data underlying a given `Attribute` is located in memory. An example of their use in `DenseInstances` is to store the column offset. `DataGrid` implementations outside of `base` won't be able to add additional fields to an `AttributeSpec` but they can:
+* Maintain local `map[AttributeSpec]int` structures to offer fast resolution.
+* Extend `AttributeSpec` to add additional fields (untested).
+
+When deciding which AttributeSpec to return, implementations should use strict equality (using `Attribute.Equals`, otherwise odd problems (like `CategoricalAttributes` having corrupted orderings) might cause odd behaviour.
+
+### `AllAttributes`
+Simply returns a copy of all of the available `Attributes`. This is used for determining compatibility with other `DataGrid` implementations, and is usually a precursor to `GetAttribute` calls. It should occur in a fixed order.
+
+### `AddClassAttribute`
+Each `DataGrid` implementation keeps track of which `Attribute`s are designated class variables. Normally, this is done using a `map[Attribute]bool` structure.
+
+### `RemoveClassAttribute`
+A call to this method means that the argument should no longer appear in calls to `AllClassAttributes`.
+
+### `AllClassAttributes` 
+This method returns every `Attribute` designated as a class `Attribute` via previous calls to `AddClassAttribute`.
+
+### `Get`
+This method takes an `AttributeSpec` and a row number and returns a slice of bytes (which can be converted to another value using `Attribute`-specific methods). At least one byte should be returned. 
+
+### `MapOverRows`
+This allows algorithms to iterate over all the rows in the `DataGrid` in whichever order is convenient for the underlying implementation. The first argument is a slice of `AttributeSpec` structures describing which fields are needed. The second argument is a function pointer which takes two arguments. The first argument of the function pointer is a slice of byte slices containing all of the binary on a given row. The second argument is a row number. The return values are a boolean saying whether the inner algorithm has terminated, and an optional error if the inner algorithm terminated with an error.
+
+### `RowString`
+`FixedDataGrid` adds a `RowString` method for easier inspection. The argument is the row number to be printed.
+
+### `Size`
+`Size` returns the current dimensions of the `FixedDataGrid`. The first value returned is the number of Attributes, the second value is the number of rows.