Implement quality of life fixes

[polsys]

• Bug fix: cond preprocessing was not done per-column; I had this in an earlier version but it was lost in some refactoring
• Bug fix: entropy estimation with both mask and cond now works
• Fixed Consider support for numpy masked arrays #16 by not adding support for masked arrays. Instead, added a drop_nan parameter. This has the benefit of working nicely also with pandas and pure Python data types.
• Fixed Support separate lags for each conditioning variable #30 by adding support for separate lags for each conditioning variable. This is done by passing a 2D array (1D arrays are not promoted for consistency).

[polsys]

This might have introduced a small performance regression on the real-world code I'm using, but it's hard to say for sure (lots of noise) and the benchmarks and integration tests show no difference. Anyways it's better than Alpha2...