Improved mmcif logging (#154)

Warnings for unknown keys are only printed once per loop, and only if a
non-empty value is stored.
Merge pull request #157 from sbliven/master

Improved mmcif logging (#154)
Implemented calculation of unique crystal contacts
Now hetatoms can be filtered by size when calculating ASAs in interfaces
Improved HETATOMs filtering for interface ASA calculation
Some refactoring and improvement of docs