change "double" to "float" in DataFormats/METReco

continuing from https://github.com/cms-sw/cmssw/pull/3439, this PR changes "double" to "float" in SpecificGenMETData

I made this PR separate from https://github.com/cms-sw/cmssw/pull/3439 so https://github.com/cms-sw/cmssw/pull/3439 can be still merged even if this PR causes an error