Add example of basic python input file

[dpgrote]

This PR adds an example using the lower level Python interface for WarpX. Here, the input parameters are set directly instead of by using the PICMI interface. This uses a fairly straightforward example to demonstrate the basic method. More advanced capabilities can be setup this way as well, for instance adding call backs to Python routines.

Related to #6149 and #5283

[dpgrote]

This will be slightly different behavior since the -q_e will be evaluated in the input file rather than in the C++ parameter parsing. This could also be
electrons.charge = "-q_e"
which would be evaluated in C++.

[dpgrote]

Because of the way this parameter is parsed, this must be a string.

[ax3l]

We could generalize libwarpx to assume Cartesian, if an integer is passed. We already do this assumption/convention in the ParmParse:

geometry.dims = 1  # Cartesian
geometry.dims = 2  # Cartesian
geometry.dims = 3  # Cartesian
geometry.dims = RCYLINDER
geometry.dims = RSPHERE
geometry.dims = RZ

[ax3l]

Discussed today in the developer meeting:

We like both and will merge both this and #6149 for now, follow-up ideas are:

• Generalize (and rename) the Bucket to support nested ParmParse options by default – remove derived classes
• With that, use the load file in 6154 to populate the nested ParmParse Bucket, so we can manipulate it.
• Enable to update ParmParse interactively through changes to Parmparse Bucket, i.e., in WarpX interactive use

With those updates, the logic here will be a little less rough (e.g., see Diagnostics) and can then be documented for users together with the example in #6149 (i.e., load file and then change parameters on "ParmParse Bucket")