Mesh refinement: multiple Poisson solvers

[SeverinDiederichs]

This PR adds multiple Poisson solvers, one for each level of MR.

Since all of the member variables are different on each level, the m_poisson_solver is now a vector over levels.
Additionally, previously only a single geometry was handed to the Fields::AllocData function, now it is a vector of over levels and the correct geometry is used.

This PR was tested by printing the dx in the creation of the Poisson solver, so each level indeed generates the correct arrays.

This PR is based on #526.

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable