[WIP] Rename current_fp/current_cp to j_fp/j_cp

[RemiLehe]

This makes the name shorter and more consistent with the rest of the code.

Note that this will break the Python access API.

[EZoni]

Just a thought, as we use upper-case E and B for the fields, should we use upper-case J for the current as well? I think it's typically written as J, not j, in textbook formulas. Also, in code, j is easy to confuse with a count index.