Separate data writing and advancing coupling operations

[IshaanDesai]

Current the function fsi_write_and_advance has two core functionalities from the typical coupling time loop: (1) writing data to preCICE and (2) advancing preCICE.
For clarity it is favourable to separate these two functionalities into separate functions" write_data() and advance().

[BenjaminRodenberg]

I think that this is definitely something we should go for in a first release of the adapter, because it defines what the user sees and this should be consistent with the overall preCICE API. I don't think we absolutely need this for the first working version. Therefore, I will add it to the v0.1.0 milestone, but not to minimal working state.