WIP: First stab at a rolling_cov_pairwise function. Fixes GH1853
TST: Added a test for rolling_cov_pairwise similar to the test for
rolling_corr_pairwise.
REF: Factored out commonalities between rolling_cov_pairwise and
rolling_corr_pairwise into one reusable function.
REF: Refactored further in preparation for accomodating ewmcov and
expanding_cov.
TST: Refactored the pairwise moment tests in order to be able to reuse them for
other moments.
TST: Added tests for ewmcov_pairwise and ewmcorr_pairwise.
ENH: Implemented ewmcov_pairwise and ewmcorr_pairwise.
TST: Added unit test for expanding_cov_pairwise based on the test for
expanding_corr_pairwise.
ENH: Implemented expanding_cov_pairwise and refactored the
expanding_corr_pairwise implementation to use _flex_pairwise_moment which gives
access to the other keyword arguments as well as allowing for different columns
in the df1 and df2 arguments.