[WIP] Rename Efield/Bfield to E/B

[RemiLehe]

This makes names shorter and more consistent.

This is a breaking change for Python users that access he E- and B-field directly by name.
Was, e.g.,:

.multifab("Efield_fp", ...)

is now:

.multifab("E_fp", ...)

For:

• Efield_...
• Bfield_...
• .._fp, ..._cp, ..._aux, ..._cax, ..._fp_external, ..._avg_fp, ..._avg_cp

[clarkse]

Looks like a decent search and replace refactor. I am happy to not have to type the word 'field' nearly as often in the future.

[ax3l]

@RemiLehe this needs a small update for merge conflicts please :)

In tests, it looks like only test_2d_embedded_circle.run is failing right now with

STEP 11 starts ...
terminate called after throwing an instance of 'std::runtime_error'
  what():  Assertion `flo * fhi <= T(0)' failed, file "/home/vsts/work/1/s/build/_deps/fetchedamrex-src/Src/Base/AMReX_Algorithm.H", line 116, Msg: "Error - calling bisect but lo and hi don't bracket a root."
SIGABRT

[dpgrote]

This should be changed to E_x_mf = warpx.multifab("E_fp", dir=0, level=0) in a later PR if not already fixed.