adds masking functionality

[derNarr]

Changes proposed in this pull request:

• adds new keyword argument to ndl learners cues_to_mask which allows to specify a set of cues that will not learn to predict themselves if they appear as cues and as outcomes.

Todos:

• At the moment the low level Cython code is duplicated and slightly changed. This should yield the smallest speed penalty as otherwise an extra condition has to be checked in the most inner loop. But if the compiler is smart enough to implement it branchless, the code should be rewritten to have no code duplication. Timings are needed here.
• Should masked be renamed to masking in the low level functions?
• split tests into tests that run on MacOS and Windows and those which run on Linux only

[coveralls]

Coverage increased (+0.6%) to 83.704% when pulling cd36607 on masking2 into a32093c on master.

[lgtm-com]

This pull request introduces 3 alerts when merging 64ac6ad into 7a488dd - view on LGTM.com

new alerts:

• 3 for Unused local variable

[lgtm-com]

This pull request introduces 3 alerts when merging cd36607 into a32093c - view on LGTM.com

new alerts:

• 3 for Unused local variable