Add hipace.initial_step option for restarting simulations

[AlexanderSinn]

This PR adds the

hipace.initial_step = 0

input parameter, which shifts the starting step to make it easier to restart simulations.

This can be used in combination with the from_file beam and laser input types to restart a simulation. To combine the diagnostic output with the previous run, it can also be useful to remove the diagnostic output of the first new step so that wont be duplicated.

hipace.initial_step = 100
hipace.initial_time = 1e-7

diagnostic.output_period = if(step==100, 0, 1)

beam1.injection_type = from_file
beam2.injection_type = from_file
beams.input_file = ...
beams.iteration = 100

laser1.init_type = from_file
laser1.input_file  = ...
laser1.iteration = 100

• 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]

Looks great, thanks! Could you give a more detailed example use case in the description?