remove mass-gap category

astrophysical categories are now BNS, NSBH, BBH. Templates greater that 3 solar mass are considered BHs.

@shasvath.kapadia would you please look over the change. This is to remove the mass gap. In summary, I have

• removed mass-gap weight - templates > 3 solar mass are now BHs
• I have updated the mean counts static file removing Counts_MassGap, and updating Counts_BBH and Counts_NSBH by distributing the mass gap weights equally between these categories.
• This change does not affect previous values significantly (unittests give roughly the same answer).
• Further fine-grained changes can be taken as a next step.

ligo/data/H1L1V1-mean_counts-1126051217-61603201.json

-{"counts_BNS": 2.11050326523, "counts_NSBH": 1.56679410666, "counts_BBH": 9.26042350393, "counts_MassGap": 2.40800240248, "counts_Terrestrial": 3923}
+{"counts_BNS": 2.11050326523, "counts_NSBH": 2.7707953079, "counts_BBH": 10.4644247052, "counts_Terrestrial": 3923}