Fixes missing var_itemsize for grid = 2.

@mdpiper sorry mark it was still failing on initialize. I ran pymt_prms_soil_ex.py do try and figure out why PRMSSoil was failing on the initialize. I discoverted that it was failing on bmi.grid_type when grid_id = 2. Two variables with grid_id = 2 had missing var_itemsize methods I assume that was the error. these two variables have grid_id, type, size, get_value functions but not units, I assume that is ok. Also I hacked the code to get past where this failed on method dataset_from_bmi_grid by adding if grid_id < 2: and the script ran to completion so I hop that I've solved the error.

PS not sure why the first set of travis runs fails but the second completes.