Allow Diagnostic output at arbitrary steps using Parser

[AlexanderSinn]

This PR extends the diagnostic output period input parameters with parser support. Because with the current functionality, = 0 means no output and = 1 means output every step, it extends quite nicely to arbitrary conditions that can then return 0 or 1, while still having the old behavior with values > 1 as an easy-to-use default.

# Current functionality:

# output nothing
diagnostic.output_period = 0
# output every time step
diagnostic.output_period = 1
# output every 10 steps and the last step
diagnostic.output_period = 10

# New in this PR:

# output only max_step
diagnostic.output_period = current_step == max_step
# same is = 10, but remove first step
diagnostic.output_period = if(current_step == 0, 0, 10)
# same as above
diagnostic.output_period = 10 * (current_step != 0)
# output every 10 steps, not necessarily the last step
diagnostic.output_period = fmod(current_step, 10) == 0
# same as = 10
diagnostic.output_period = fmod(current_step, 10) == 0 or current_step == max_step or current_time == hipace.max_time
# nonuniform output period
diagnostic.output_period = if(current_time <= 350, 2, 10)

• 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]

Thanks for this PR! Could you update the documentation accordingly? It should contain a description of the new behaviour, but for the examples I think it's OK to link to the PR description.