little cleanups

Whenever `$am` is used for a `Batch` object, change it to `$batch`.

Remove the usage of `unknown => '='` or other options that are passing
what is already the default value.
remove unneeded variable

I was going to rename `datatocontext` to something more descriptive,
but in the process I found that it wasn't accessed anywhere. Since
there is no current use of it, remove it.
fix use of 'data' throughout

`Data` was originally used throughout the code base to mean training
items. Remove this usage entirely, (especially) including
documentation.

This required the change of one public facing method: `test_in_data`,
was changed to `test_in_train`.

Update `Changes`.
use accessors within Result

Much of the code in `Result` was referring directly to `$self->{state}`
instead of using the appropriate accessor, so fix this.
scrub code of 'variables'

The use of `variable` has been replaced everywhere with `feature`,
and `var` similarly with `feat`. This has one public-facing change,
which is that gang effect structures now have a `features` key instead
of a `vars` key. Update `Changes`.
New CPAN release of Algorithm-AM - v3.02

    test_in_data in Result changed to test_in_train
    'vars' in gang_effects return structure changed to 'features'