MIDAS data

[jstriaukas]

Create functions that compute MIDAS data lags based on times data inputs (times series + a vector of dates for that particular times series)

Example:
https://github.com/jstriaukas/midasml/blob/master/R/date.functions.R

[slim-patchy]

Code introduced as https://github.com/jstriaukas/midasmlpy/blob/master/midasmlpy/date_functions.py.

[jstriaukas]

Data files generated from R package for test code has been uploaded to https://github.com/jstriaukas/midasmlpy/tree/master/midasmlpy/data_files

[jstriaukas]

Test functions added.