Replaced any TMatrix and TVector by SMatrix and SVector in MET significance code

Removed any instance of TMatrix and TVector in benefit to SMatrix and SVector in the MET significance code and MET code, used to keep the covariance matrix.
An extra typedef has been declared in DataFormat/METReco/interface/MET.h to standardize the definition of the MET covariance matrix.

This PR is necessary to complete the PR https://github.com/cms-sw/cmssw/pull/5388