homogenization of saving intervals

[tommy-engels]

I suggest to create a small module that takes care of the saving conditions for each task.
There is currently only saving the fields, but in the future, saving statistics and the like will be added.

We have the following conditions for saving: (some of which will be in the future)

• every N time steps
• every T physical time units
• but not before the first N0 time steps
• and not before the first T0 physical time units
• at the beginning and end of the simulation
• (?) maybe in the future, if an external command is received

This should be connected to an "intelligent" dt, which ensures we do not jump past the saving times and if necessary adjusts dt to precisely land on this instant.