add helper function that finds SARAH symbol definitions

from ParameterDefinitions
adding wrapper functions Sec() and Csc()
reuse new helper function to avoid code duplication
Bugfix: generalize calculation of gauge couplings at the low-scale

This generalization correctly handles the case where the rho parameter
is not 1. Fixes #1
store MZDRbar in the class to avoid recalculation
Merge branch 'development' into bugfix-low-scale-gauge-couplings
Merge branch 'bugfix-low-scale-gauge-couplings' into development