Added the following:
- Covariance matrix function that uses all and part of the data
- Rolling covariance matrix function

Code also allows computation of log returns or simple diffs, n lags forward or backward
Added the following:
- Covariance matrix function that uses all and part of the data
- Rolling covariance matrix function

Code also allows computation of log returns or simple diffs, n lags forward or backward
Merge remote-tracking branch 'origin/master'