Improve initial state configurability

[jbreue16]

As per our last dev call, we decided to improve and expand the initial state configurability:

• INIT_STATE can be specified on unit operation level for the full state, this should also be possible on particle level.
• Enable radially resolved INIT_CP, INIT_CS, add corresponding multiplexing

[schmoelder]

I'm happy to offer any kind of support, e.g. brain storming / developing user-stories / interface design / testing etc. Please let me know.

Edit: See also: #183