field.where functionality

[atbenmurray]

exetera needs a where function that carries out the numpy where functionality. This should probably be implemented in the form of exetera.where(a_field = 'foo', 'bar', a_field).
We could also potentially expose this functionality in other ways, such as a_field.where() although that doesn't mirror the numpy api

a = exetera.where(b == 2, 1, 0) # memory field
df['a'] = exetera.where(b == 2, 1, 0) # hdf5 field