Implement operation gradient in Generic module

[vidulejs]

Implemented operation gradient; for volume & surface scalar fields in the new Generic module. The code closely follows PressureGradientFull.C. The gradient operation on a scalar field will result in a vector field. Therefore, when operation == 'gradient', the dataType_ will become vector. The user has to make sure it's also the case in the preCICE config.

For that reason I implemented a new check in Interface::addCouplingDataWriter, where it will find if there's a vector/scalar data mismatch between the preCICE config and the adapter. If the user configured operation gradient, they will get a more specific error:

Error in the preCICE adapter: 
Data dimension mismatch for field "PressureGradient". The field is defined as
scalar data in the preCICE configuration, but the data is vector in the adapter.
Please check your preCICE configuration. If you are trying to couple the gradient
of a scalar field, make sure the data is defined vector in preCICE config.

Previously the error would be thrown by preCICE and say that the number of vertices is incorrect.

TODO list:

• I updated the documentation in docs/
• I added a changelog entry in changelog-entries/ (create directory if missing)

[vidulejs]

Unlike PressureGradientFull, I don't store the resulting volVectorField as a member variable. I'm using Foam::tmp<volVectorField> to temporarily allocate it and automatically deallocate it once it's out of scope. Would this be a performance problem - allocating and deallocating memory for each write? Alternatively, we could create a shared buffer to store the scalar field gradients if configured.

[MakisH]

Discussed in a meeting: This should indeed have performance implications, better allocate in the constructor and reuse.

[MakisH]

Looks good overall, I have not tested it.

[MakisH]

Discussed in a meeting: This should indeed have performance implications, better allocate in the constructor and reuse.