Add MPI extension allowing NamedArray and NamedTensor to be used in MPI primitives directly.

[jack-dunham]

This is a small extension allowing one to do things like:

buf = rand(Index(2), Index(2)) # create an ITensor
MPI.Irecv!(buf, comm)

without first having to unwrap to the data array.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.11%. Comparing base (c535044) to head (778846c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #258      +/-   ##
==========================================
+ Coverage   78.09%   78.11%   +0.02%     
==========================================
  Files          30       31       +1     
  Lines        1817     1819       +2     
==========================================
+ Hits         1419     1421       +2     
  Misses        398      398              

Flag	Coverage Δ
docs	26.26% <0.00%> (-0.03%)	⬇️

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[mtfishman]

After an offline discussion we decided to hold off on this for now (there are some potentially tricky things with making sure the index names of the sender and receiver are aligned properly which this implementation doesn't account for).