Fix intergrid 'maps' default argument

With the default argument of Integrid's __init__ being `maps=[]` the
interpolation failed, since the instance variable `_lo` was never set.
The default value of `maps` has been changed to a list of `None`s,
matching the dimensions of the `lo` argument.
Change version in intergrid
Merge pull request #2 from leifdenby/master

Fix intergrid 'maps' default argument