Tweak HLT1 dimuon thresholds

There are a couple of changes we could make to the thresholds of the HLT1 dimuon lines to ease the physics performance comparisons with Allen:

1. Cut explicitly on pT in the low-mass line; this is currently set to zero, we could align it with the cut in reconstruction
2. Correct (?) the cuts in the high-masss line; these currently require p_{\mathrm{T}} > 6 GeV and p > 300 MeV, are the thresholds the wrong way around?

What do we think?

/cc @cprouve (as author of the lines) @thboettc (as responsible for aligning the thresholds)