Tidy up (un)rolling

[thisac]

Context: Rolling/unrolling/space-unrolling wasn't always working as one might intuitively expect. This PR tweaks the (un)rolling a bit and also fixes a few issues.

Description of the Change:

• Rolling and unrolling now only happens in place. It has previously also been returning the (un)rolled circuit, but this was never used.
• Fixed bug when trying to unroll an already unrolled program with a different number of shots.
• Fixed bug with vacuum modes missing.
• Tidied up some docstrings.

Benefits: Things look and work better.

Possible Drawbacks: (un)rolling no longer returns the (un)rolled circuit. It only happens in place.

Related GitHub Issues: None

[codecov[bot]]

Codecov Report

Merging #702 (dee6998) into master (1948e33) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #702      +/-   ##
==========================================
+ Coverage   98.69%   98.71%   +0.02%     
==========================================
  Files          75       75              
  Lines        9212     9217       +5     
==========================================
+ Hits         9092     9099       +7     
+ Misses        120      118       -2     

Impacted Files	Coverage Δ
strawberryfields/tdm/tdmprogram.py	99.16% <100.00%> (+0.87%)	:arrow_up:

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