Rolling rework

Bert Palmrequested to merge
rollingRework into develop

Clarified and cleaned up the Rolling. Now we can have backwards rolling and step-size rolling. Also passing a mask where to skip calculation is possible. This also fixes the ramp-up and ramp-down issues for variable windows (eg. window='4d').

all pandas.rolling native features are implemented and a Rolling() bject is returned as in pandas.

# pandas
r = s.rolling(window='4d')
r.sum()
r.apply(fancyFunc)

# custom
r = customRoller(s, window='4d')
r.sum()
r.apply(fancyFunc)

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