Enable different boundary conditions for different fields on different boundaries

[anne-glerum]

To address #1560. No need to look yet, I just want to see what the testers say.

Notes to self:

• Plugins still need to supply values for all fields
• Remove unused functions
• Model operator option add is only option allowed in new format
• Only one model operator is allowed per plugin, even though the plugins can be active on multiple boundaries for different fields
• Input separators could be better

For all pull requests:

• I have followed the instructions for indenting my code.

For new features/models or changes of existing features:

• I have tested my new feature locally to ensure it is correct.
• I have created a testcase for the new feature/benchmark in the tests/ directory.
• I have added a changelog entry in the doc/modules/changes directory that will inform other users of my change.

[tjhei]

We often use "prescribed" instead of "fixed" I think. Think about which one makes more sense?

[anne-glerum]

I like prescribed more, but in the prm file and in the original plugin interface, fixed is used, e.g. Fixed composition boundary indicators and fixed_composition_boundary_indicators, so I stuck with that.

[anne-glerum]

Hi @gassmoeller, before I continue with this PR, I'd like your input. Does the PR go in the right direction? If so, what is specifically bothering me is:

1. The boundary composition plugins still need to specify boundary conditions for all fields, even if not all fields are prescribed. I can adapt the plugins; adding a function to interface.cc that returns on which boundary for which fields a plugin is active would make it easier.
2. When the new feature is used, the model operator functionality is lost (i.e. the only option is add). Shall I add in another entry that specifies the model operators? E.g. <boundary_name>; field_name_1|field_name_2|field_name_5 : plugin_name_1|plugin_name_2 & add|max. It won't improve the readability...
3. The input separators could be changed. Only the , separating entries needs to stay to keep the current format working.

[anne-glerum]

After discussing with @gassmoeller, I have updated the input format. In addition, all the boundary composition plugins now take into account how many compositional fields they are supposed to prescribe on each boundary (a test is added for each changed plugin). The only allowed operator for the new format is still add.

I think this is now ready for review. I have one question though: What to do with the old old input parameter Model name? At the moment, I've left it in, but it will not work properly. Is it time to remove it?

EDIT: it seems I broke something today, let me fix that first.

[anne-glerum]

/rebuild

[anne-glerum]

Okay @gassmoeller, this is now ready for review.