Multigrid config

[yhmtsai]

This pr adds the config interface for multigrid and coarsening method.

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[MarcelKoch]

This looks mostly good, but I have two requests:

• don't provide configuration for FixedCoarsening, it doesn't have any meaningful parameters for the configuration
• don't provide the selector parameter for the multigrid. They are too advanced to be used frequently, so we can take a bit more time to find a representation in our config file for them.

[MarcelKoch]

Without the array, the fixed coarsening is pointless. I would also suggest, that we shouldn't support this type of parameter in our config anyway, since they very explicitly depend on the concrete sizes of the matrices. Those are not (and probably should not) be available in the config, so we can't have parameters that depend on them.

[MarcelKoch]

I'm not sure this key is understandable. In general, I would suggest to not support the level/solver selector at the moment. It is again something that is very complex to use, but has very few potential use cases. I would not expect our users to actually use this.
So I would give this more thought and add it in the next release.

[yhmtsai]

I can provide another default option without settings, which I think should be more practical for most cases.
use mg_level.at(level) when level < vector_len and reuse the last one if level >= vector_len.
It may be more suitable in the default selector not in config

[MarcelKoch]

That doesn't help. The selector should not be part of this release. It's still too complex with very little use cases.

[yhmtsai]

I have created #1611 and remove the selector now

[MarcelKoch]

without the selector, this should not use vectors. Same for the other instances here.

[yhmtsai]

By default, if user gives the enough mg_level, which less than the generated levels (or limited max_mg_level).
Multigrid will use each for each level

[MarcelKoch]

I would still advocate to only allow a single value here. How many mg_levels users need to provide should again depend on the matrix, so it would be unknowable for the configuration.
Also going to multiple levels later would be backwards compatible, so there would be no issue.
(same goes for the other vector parameters here)

[yhmtsai]

@thoasm the documentation relies on two parts

1. general rules in parse of config.hpp: it gives the idea about different type of setting.
2. detail option and setting in factory parameters: aim for same support but little difference between code and config

@MarcelKoch I have removed the selector, but I still keep the vector input. User can still use it by one argument. Default selector still works normally when they give many. They can also input the selector after file config otherwise.