Fluent UDF variable names should be consistent with preCICE API names

[IshaanDesai]

The boolean variables defined by the scheme file init-fsi.scm which are modified in the source files have names like udf/convergence, udf/iterate. These names should be more consistent with the main preCICE API to provide a better understanding of the adapter workflow.
Possible changes are:
udf/convergence --> udf/isConvergenceAchieved
udf/iterate --> udf/isImplicitIterationComplete
udf/ongoing --> udf/isCouplingOngoing
These variables are critical as they act as boolean signals between preCICE and FLUENT.

[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.