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.