Add writing of fixed point types to write_array

[rowleya]

Write array doesn't currently work for fixed point types. This could be made to work through data conversion.

[andrewgait]

Would we want to do this for all fixed point types, or just for the ones that are used regularly (i.e. S1615 and U032)?

[rowleya]

Ideally it would work for anything the DSG supports. I would guess this could do a type conversion to integer first to make the transition; since this is numpy, it should be fast enough...