Add possibility to input multiple beams at once

[AlexanderSinn]

This PR adds the possibility to input multiple beams from the same file with the same input parameter instead of specifying them one by one.
Old, but still possible:

beams.names = beam1 beam2
beam1.injection_type = from_file
beam1.input_file = beam_%T.h5
beam1.iteration = 0
beam2.injection_type = from_file
beam2.input_file = beam_%T.h5
beam2.iteration = 0

New:

beams.names = beam1 beam2
beams.all_from_file = beam_%T.h5
beams.iteration = 0

It's also supported to specify iteration, plasma_density and file_coordinates_xyz for all beams at once.

• 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]

Tested and works well. Thanks!