Fix double inversion of MET covariance matrix

Bug was inserted during https://github.com/cms-sw/cmssw/pull/5417,

MET covariance matrix was inverted twice instead of once, now corrected. Impacts MET significance results only.
fabs corrected to std::abs

In the old code, that line only needed the determinant w/o any inversion of metmat, which is done somewhere else. When the move from TMatrix to SMatrix was done, I did not noticed that the Det function was doing both, while Det2 is only computing the determinant and then was the matrix to be used.