Fix dask+atom_slice

[sroet]

Due to an discrepancy between DaskFrequency and it's parent class (ContactMap) __init__() , we tried to convert indices back to unsliced two times:

• DaskFrequency calls the super.__init__:
• ContactFrequency.__init__() converts to sliced indices, calls _build_contact_map and converts back to real indices

However DaskFrequency._build_contact_map constructs new ContactFrequency objects and sums them (in unsliced indices), so after this the ContactFrequency.__init__() tries to convert these already correct indices back again.

In order to solve this I moved the conversion from sliced to regular indices as part of the _build_contact_map routine of ContactFrequency.

fixes #51

[dwhswenson]

LGTM. Merging. (We might consider dropping/changing the comments on n_workers=4 in tests though, since our current setup is much improved on the thing we're referencing -- the reference probably only serves to create confusion.)