Fix #99 Push/pop Timer name prefixes

[vasdommes]

Summary:

• Rewrote ScopedTimer to ensure that nested timers will have correct prefixes, see changes in Timers.hxx. Also added asserts to ensure that nested timer always stops before parent.
• Added some new timers, e.g. "sdpb.solve.run.iter_XXX" for each solver iteration.
• Renamed some timers, e.g. for sdp2input.
• Fixed Timer::elapsed_milliseconds(): show correct time even if timer is still running.

ScopedTimer now temporarily adds its name to Timers::prefix.
Example:

void f()
{
  Timers timers(false);
  Scoped_Timer root_timer(timers, "root"); // "root"
  Scoped_Timer x_timer(timers, "x"); // "root.x"
  x_timer.stop();
  Scoped_Timer y_timer(timers, "y"); // "root.y"
  {
    Scoped_Timer yy_timer(timers, "yy"); // "root.y.yy"
  }
  Scoped_Timer z_timer(timers, "z"); // "root.y.z"
}

The only change to SDPB behaviour is timer names printed to profiling.* in debug mode. Beside that, this PR is mainly about improving code quality.