Encapsulate field IO, first step to multiple slice IO

[MaxThevenet]

This PR is a first step to enable multiple IO slices. It removes details about field diagnostics (2D XZ slice or full 3D XYZ array) and puts it in a separate class FieldDiagnostic. The main changes are:

• a new class FieldDiagnostics was created, that contains the IO data + metadata
• Fields::m_F is now a DUMMY multifab (data not allocated) only used for its metadata, while it used to be holding the IO.
• Fields now has a new member FieldDiagnostic m_diags.
• The slice type is extended from xz only to xz or yz.

input parameters hipace.output_slice and hipace.3d_on_host have been removed. The slice type is now given by

diagnostic.diag_type = xz # or yz or xyz

As before, only the central slice is available.

• 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

[SeverinDiederichs]

Looks good! openPMD works fine, I tested it on GPU, single precision, with xyz, xz, and yz slices.