Add concatenate tests.
Add concatenate matrices API with axis.
Fix tests and refactor extent translation.
Refactor concatenate mapper and extent translation

This fixes the negative upper left boundary bug in translate extent that
occurs when tmp_ul is negative. The negative dimension in the upper left
coordinate will instead be set to 0.

The column logic and 1d if branch are replaced with a tight if, elif,
else branch that sets the row in result to either data_a, data_b, or
data_a + data_b (using np.concatenate).
Merge branch 'add-concatenate-to-builtins' of git://github.com/rgardner/spartan into rgardner-add-concatenate-to-builtins

Conflicts:
	tests/test_builtins.py
Merge branch 'rgardner-add-concatenate-to-builtins'