Core: how to improve Method class for single-iteration use case?

[GPMueller]

The Problem with the SingleShot API function and PlayPause with a single iteration is that a Method instance has to be created. When e.g. sampling with single iterations (or only a few), the resulting overhead is significant! How could this be avoided?

The main problem with keeping around an instance of Method while it is potentially unused is that this may use a significant amount of memory. Depending on system size, this may be unacceptable.

Currently, my best idea is:

• add an API function, e.g. Simulation_Start_Single(state *, const char * method, const char * solver, int, int)
• this would create a Method, which would be stored in the State and block the corresponding image/chain from other simulations
• then calling Simulation_SingleShot would not have any unusual overhead
• this could also be used to avoid all the output currently generated from repeated single iterations, see also #417
• add an API function, e.g. Simulation_End_Single, which would delete the Method and free the blockade

[GPMueller]

Changed the API and implemented the better single-shot procedure with 06766639b24d2425c588564f95be4ba595be7da1 on feature-improved-api.

[GPMueller]

Merged into develop with eb841c5bd39941d4c437895845969c00ff224f10.