Add program code generator function

[thisac]

Context: The convert-to-python-code functionality in SF interactive should be moved to SF.

Description of the Change: Added a generate_code function to io.py that generates SF code from a Program or a TDMProgram, returning the code as a string. If an engine is passed, the engine-related code is also serialized.

Benefits: It's possible to convert a program/engine into code directly via SF.

Possible Drawbacks: None

Related GitHub Issues: None

[codecov[bot]]

Codecov Report

Merging #496 (79df368) into master (98f557a) will increase coverage by 0.00%. The diff coverage is 98.07%.

@@           Coverage Diff           @@
##           master     #496   +/-   ##
=======================================
  Coverage   97.86%   97.86%           
=======================================
  Files          70       70           
  Lines        7316     7368   +52     
=======================================
+ Hits         7160     7211   +51     
- Misses        156      157    +1     

Impacted Files	Coverage Δ
strawberryfields/io.py	96.85% <98.07%> (+0.45%)	:arrow_up:

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[antalszava]

Thanks for the clarification @thisac! The main piece missing for me was that internally we'd already have the numeric values for gate parameters (which actually makes a lot of sense now) and we'd like to convert those numeric values to expressions that contain np.pi.

[antalszava]

[ch1778]