Add marginalisation for eccentricity

[ivalaginja]

This is not really needed, but for consistency we should add the marginalization of the eccentricity and also output that to the report. This will only happen if the user choses the grid selection fit_ecc (not even needed for fit_all), so this implementation is really low priority.

[hrwakeford]

This issue is linked to #16 which requires more modification due to changes in planetary orbit and timing parameters associated with eccentric planetary orbits.