Tweaks from componentizing

Created by: mdpiper

This PR contains a bunch of minor changes to the PRMS Surface component BMI that resulted from transforming it into a pymt component. BMI and pymt are slightly out of sync. (For example, the get_grid_size function is planned to be deprecated in BMI, but in pymt, it has already been replaced by get_grid_node_count.) The changes made here to the BMI implementation are minor, but the component raises exceptions in pymt without them.

The BMI tweaks altered the expected values of the tests, so they've been updated, as well. This fixes #17 (closed).