faster gaussian compiling

[jakeffbulmer]

to speed up large circuits with a lot of gates, we wish to avoid a full size, dense matrix-matrix multiplication upon application of each gate.

In this PR we will try to avoid this. My first attempt is to use scipy.sparse.csr_matrix to reduce the complexity of the matrix-matrix multiplications. However, we may also look into hardcoding the matrix-matrix multiplication.

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[codecov[bot]]

Codecov Report

Merging #598 (d9dceab) into master (7984174) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #598   +/-   ##
=======================================
  Coverage   98.45%   98.46%           
=======================================
  Files          76       76           
  Lines        8758     8780   +22     
=======================================
+ Hits         8623     8645   +22     
  Misses        135      135           

Impacted Files	Coverage Δ
strawberryfields/compilers/gaussian_unitary.py	100.00% <100.00%> (ø)

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[jakeffbulmer]

because of scipy sparse overheads, I think this change would only actually help for systems with several hundred modes or more!