Set initial simulation time step earlier

[HomesGH]

Description

Until now, the initial time step is set at the end of Simulation::prepare_start(). At this point, the plugins are already initialized which could lead to a wrong initializations of them. An example is the MMPLDWriter, which uses the number of simulation steps to calculate the file header of the .mmpld files during initialization.

The simulationTime (physical time, not time step) can be either changed by the config (currenttime) or the checkpoints. The calculation of the initial time step is now moved to right after the simulationTime was changed the last time before the actual simulation starts.

Test:

• [x] Run the surface tension example with MMPLDWriter. Observe and compare mmpld file with mmpld tool.

Result of test:

• master: warning: dead data trailing header: position after reading frame table (156) is not the start of the first frame (8156)
• This PR: No warnings/errors

[HomesGH]

Did you do the test you announced in the PR description? ;)

Yes, I just did it. See Description of PR for details