ENH: Add Stairs.to_pickle and Stairs.from_pickle #97
Description
Describe the solution you'd like
Should work with a straightforward application of python's pickle module.
Compare https://pandas.pydata.org/docs/reference/api/pandas.Series.to_pickle.html with pickle.dump (https://pandas.pydata.org/docs/reference/api/pandas.Series.to_pickle.html) to assess which parameters might be helpful.
Pandas uses pd.read_pickle, but we can just implement Stairs.from_pickle