Workaround for the Weinberg angle in the MRSSM in SARAH 4.2.2

SARAH 4.2.2 defines Weinberg angle as

  {ThetaW, { Description -> "Weinberg-Angle",
             DependenceNum -> None,
             Value -> 0.511621}},

However, we currently need its analytic expression in terms of MW and
MZ.  This workaround puts the analytic expression back into
SARAH`ParameterDefinitions.