Add option to limit number of buffered beam slices

[AlexanderSinn]

This PR adds options

hipace.comms_buffer_max_leading_slices = 10
hipace.comms_buffer_max_trailing_slices = 10

to limit the amount of beam/laser slices that can be stored on any one rank. This is necessary for large beams/laser boxes that do not fit into the memory of a single rank as the fully asynchronous penalization without these parameters may put most of the data on to one rank. If the simulation has more timesteps than slices, then these parameters need to be set such that all ranks together can accommodate all slices to avoid a deadlock.

• 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

[MaxThevenet]

Is this ready for review? If so, great! Could you add a description and show/discuss how it works and why it matters?

[MaxThevenet]

Looks great, thanks! See comments below.