Implementation of n-dimensional Gaussian constraints

Summary:
Implementation of n-dimensional Gaussian constraints on fit parameters
Closes T214

Test Plan: Tests with toys show spreads and correlations between parameters that are consistent with the constraints applied

Reviewers: tlatham

Reviewed By: tlatham

Subscribers: tlatham

Tags: Laura

Maniphest Tasks: T214

Differential Revision: https://phab.hepforge.org/D93