Darkscalar: Addition of B mass constraint to dtf (Superceeds MR !2454)

AP for B2KDarkScalar, DarkScalar2hh (hh = KK, pipi)

Changes: Edit dtf.py s.t. the decay tree fitter now places a mass constraint on the B

Superceeds MR!2454

• B^+ \rightarrow K^+ \chi( \rightarrow hh), where \chi represents a long-lived dark scalar
• hh: K^+K^- and \pi^+\pi^-
• 2018 Run 2 data
• Using stripping lines: StrippingB2KX2KKDarkBosonLine and StrippingB2KX2PiPiDarkBosonLine
• WG: QEE
• Code is based on APrimetoee AP (Dark photon AP)
• For details of how this code works see the README