Remove matmuleinsum backend

[stavros11]

As discussed in #398 this removes the matmuleinsum backend and renames the existing backends as follows:

• qibotf: The old custom backend which now loads the custom operators from the external qibotf library.
• numpy: The numpy defaulteinsum backend.
• tensorflow: The old defaulteinsum backend which uses the numpy implementation replacing np.einsum with tf.einsum.

[scarrazza]

@stavros11 thank you, looks pretty good.
Please above minor comments.

[codecov]

Codecov Report

Merging #404 (fc9dffd) into simtf (8cdcdcd) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             simtf      #404    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           76        76            
  Lines        12401     12265   -136     
==========================================
- Hits         12401     12265   -136     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/qibo/backends/__init__.py	100.00% <100.00%> (ø)
src/qibo/backends/abstract.py	100.00% <100.00%> (ø)
src/qibo/backends/einsum_utils.py	100.00% <100.00%> (ø)
src/qibo/backends/numpy.py	100.00% <100.00%> (ø)
src/qibo/backends/tensorflow.py	100.00% <100.00%> (ø)
src/qibo/core/circuit.py	100.00% <100.00%> (ø)
src/qibo/core/distcircuit.py	100.00% <100.00%> (ø)
src/qibo/core/fusion.py	100.00% <100.00%> (ø)
src/qibo/core/gates.py	100.00% <100.00%> (ø)
src/qibo/models/variational.py	100.00% <100.00%> (ø)
... and 17 more

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[stavros11]

Thanks for the fixes, I checked using the test_prints.py script and I think there aren't any leftover "defaulteinsum" and "matmuleinsum" references in the source code or the examples.

I will continue working on the points you wrote in #398 and particularly bring all the tests in the tests_new format.