Variable plasma density

[AlexanderSinn]

This PR addresses #597

<plasma name>.density is now replaced with <plasma name>.density(x,y,z) which can take a function of x, y and z.
x and y coordinates are taken from the simulation box and z = time * c.
The density gets recalculated at the beginning of every timestep.

If specified as a command line parameter, quotation marks must be added:
"<plasma name>.density(x,y,z)" = "1.".

• 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]

Very nice, thanks for this PR!