Changed typo in regression_gaussian_process_modular.py 

First attempt to pull request in github. :)

Changed a typo in regression_gaussian_process_modular.py from GP_RETURN_MEAN to GP_RETURN_COV

The covariance returned has negative numbers and I have not found the reason. It might be a problem in the GP regression algorithm and a unit test might be required.

P.S. This time to the right branch - develop!