Space unrolling for TDM programs

[thisac]

Context: This PR adds support for using a TDMProgram to construct TDM circuits with Fock measurements and multiple loops (e.g. TD3). The major change is needed support for space-unrolling.

Description of the Change:

• Updates TDMProgram to support TDM circuits with Fock measurements.
• Adds a method for space-unrolling TDM circuits (works for all TDM circuits).
• Adds a helper function to calculate the mode indices and number of concurrent modes in a TDM program.
• Updates the engine to support compiling TDM programs with the passive compiler.

Note: when space-unrolling a circuit, subsystems need to be added to the TDMProgram. These are removed when rolling back the circuit.

Benefits: TDM programs with Fock measurements can be constructed (via TDMProgram) and all TDM programs can be space-unrolled.

Possible Drawbacks: None

Related GitHub Issues: None

[codecov[bot]]

Codecov Report

Merging #601 (6255187) into master (cda84e4) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #601   +/-   ##
=======================================
  Coverage   98.55%   98.56%           
=======================================
  Files          77       77           
  Lines        8857     8893   +36     
=======================================
+ Hits         8729     8765   +36     
  Misses        128      128           

Impacted Files	Coverage Δ
strawberryfields/engine.py	96.49% <100.00%> (+0.07%)	:arrow_up:
strawberryfields/tdm/tdmprogram.py	98.15% <100.00%> (+0.30%)	:arrow_up:

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