Fix warnings during MMC package compilation
This MR is meant to fix warnings during MMC compilation package due to e.g:
- warning: ATLAS coding standards require that variable name 'm_met_coscovphi' not start with 'm_'.
- warning: Use of static expression 'MissingMassCalculator::s_ter_sigma_par[1][5][1]' of type 'double' within function 'MissingMassCalculator::MissingMassCalculator()' may not be thread-safe.
Edited by Antonio De Maria