Fix to vexp CLI

[brudfors]

• inplace argument removed as no longer in spatial.exp
• displacement option should be given as bool, not char

with these fixes, converting a velocity field to a displacement field works well for me.

[balbasty]

Thanks @brudfors ! (I am not sure why the displacement stuff was not working, since (type.lower()[0] == 'd') should yield a bool, but anyway the change makes it more readable)