Renaming the hafnian library to The Walrus

[josh146]

Description of the Change:

This PR makes the following changes:

• Renames the Python library to 'The Walrus', and updates all according docstrings, imports, tests, and documentation.

• Renames the C++ header library to libwalrus, and updates the namespace, filenames, tests, and makefiles.

• Moves the C++ source code from src to include, to better reflect the convention for a header only library

• Moves the C++ tests into their own top level folder (tests)

• Since we now only have one library .so file, I have removed the hafnian/lib subfolder, and now the library is available at thewalrus/libwalrus.so. As a result, the import paths have become simpler:

  from hafnian.lib.libhaf import haf_real

  is now

  from thewalrus.libwalrus import haf_real

Benefits: Simplifies the library structure

Possible Drawbacks: n/a

Related GitHub Issues: n/a

[codecov]

Codecov Report

Merging #34 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #34   +/-   ##
=======================================
  Coverage   95.21%   95.21%           
=======================================
  Files           9        9           
  Lines         627      627           
=======================================
  Hits          597      597           
  Misses         30       30

Impacted Files	Coverage Δ
thewalrus/reference.py	98.73% <ø> (ø)
thewalrus/__init__.py	100% <ø> (ø)
thewalrus/_version.py	100% <ø> (ø)
thewalrus/samples.py	85.35% <ø> (ø)
thewalrus/quantum.py	97.82% <ø> (ø)
thewalrus/_hafnian.py	100% <100%> (ø)
thewalrus/_hermite_multidimensional.py	98% <100%> (ø)
thewalrus/_torontonian.py	100% <100%> (ø)
thewalrus/_permanent.py	100% <100%> (ø)

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