Bugfix: thread-safety in the calculation of the Higgs pole-masses

The two-loop routines from Slavich are not thread-save.  Therefore, if
these two-loop routines are enabled (via UseHiggs2LoopMSSM or
UseHiggs2LoopNMSSM) the functions

  calculate_MAh_pole
  calculate_Mhh_pole

have to be calculated sequentially in the same thread (= main thread
here).
rename function