use std::chrono::high_resolution_clock in Stopwatch

std::chrono::steady_clock and std::chrono::high_resolution_clock are
both found in the final C++11 standard, but the latter is offered also
by gcc prior to 4.7.  Furthermore, its name sounds better.
Workaround: Make CalculateScaleFromExprSymb[] work for all Mathematica versions

The current workaround simply strips the ConditionalExpression[]
from the solution.  This will work even from Mathematica versions
which have the following misbehaviour:

  In := Solve[(BETA[g1]-BETA[g2]) Log[x/a] == g1-g2, x, Reals]

  Solve::nsmet: This system cannot be solved with the methods
available to Solve.

  Out = Solve[(BETA[g1]-BETA[g2]) Log[x/a] == g1-g2, x, Reals]
Merge branch 'master' of ssh://pktlogin.phy.tu-dresden.de/soft/users/theory/FlexibleSUSY