Remove last else-cases in FF and CHT

[maxfirmbach]

Loading several modules e.g. FF and FSI results in an error message (see issue #196).
The last else-case in the FF- and CHT-modules for writing and reading participants is deleted,
which should provide a first fix. Now the branching should be consistent in all three available modules (FSI, FF, CHT).

[MakisH]

Thank you very much for the contribution, @maxfirmbach!

I went ahead and added a similar error message that checks across modules. Scenarios:

• The user does not specify any read or any write data --> Nothing is happening, preCICE works, no data exchange, no warning. (do we want to change this?)
• The user mistypes a data type --> Error message.
• The user forgot to enable a module --> Error message.
• The user has extended the adapter and now has two modules defining the same read/write data set --> Error message (to not rely on picking the last occuring one).

To implement this error, I had to change the return type of the addWriters and addReaders methods from void to bool and count in how many modules a dataset is found.

@davidscn please cross-check my changes, at least by looking at the diff.

[davidscn]

The user does not specify any read or any write data --> Nothing is happening, preCICE works, no data exchange, no warning. (do we want to change this?)

From what I understand you raise an error in case no reader or no writer was specified. This disables unidirectional coupling. We should only raise an error message in case the reader and writer are in total zero?!

The user mistypes a data type --> Error message.

✔️

The user forgot to enable a module --> Error message.

✔️

The user has extended the adapter and now has two modules defining the same read/write data set --> Error message (to not rely on picking the last occuring one).

✔️

[MakisH]

@davidscn thanks again for your suggestions, I am ready for another review.

As a summary, I took the original suggestion (mostly a work-around) and changed every module to handle possible related errors.