Implement multifab wrapper functionality from WarpX (#370)

This adds the code from the WarpX fields.py that allows global indexing
of MultiFabs. This is done by adding `__getitem__` and `__setitem__`
methods for the MultiFab object.

Note that this incorporates the changes from WarpX PR #4370 that had not
been merged in.

This currently only does the get/set without including the ghost cells.
What is needed is a way of specifying inclusion of the ghost cells.
Perhaps this?
```py
mf = MultiFab()
mf[:,5,6]  # without ghost cells
mf[:,5,6] = 2.  # without ghost cells
mf.with_ghosts([slice(None),5,6])  # with ghost cells (in the first dimension)
mf.with_ghosts([slice(None),5,6], 2.)  # with ghost cells
```
Maybe this, using imaginary numbers to refer to ghost cells?
```py
mf[-1j,5,6]  # the first ghost cell on the lower boundary
mf[+1j,5,6]  # the first ghost cell on the upper boundary
mf[(),5,6]  # the full first dimension including ghosts
mf[:,5,6]  # the first dimension without ghosts
```
I prefer this method.

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

docs/source/usage/compute.rst

@@ -48,6 +48,14 @@ This is how to iterate and potentially compute for all blocks assigned to a loca
          :start-after: # Manual: Compute Mfab Detailed START
          :end-before: # Manual: Compute Mfab Detailed END
 
+   .. tab-item:: Global
+
+      .. literalinclude:: ../../../tests/test_multifab.py
+         :language: python3
+         :dedent: 4
+         :start-after: # Manual: Compute Mfab Global START
+         :end-before: # Manual: Compute Mfab Global END
+
 
 For a complete physics example that uses CPU/GPU agnostic Python code for computation on fields, see: