Usability: Warning when user calls "return" from their autoqasm entry point function

[laurencap]

Issue #, if available:

Description of changes:

Warn if the called function returns a value

Testing done:

New unit tests

Merge Checklist

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[codecov]

Codecov Report

Merging #661 (378aeb9) into feature/autoqasm (10a80e2) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@                 Coverage Diff                  @@
##           feature/autoqasm     #661      +/-   ##
====================================================
- Coverage             99.92%   99.92%   -0.01%     
====================================================
  Files                   148      149       +1     
  Lines                  8764     8752      -12     
  Branches               1879     1872       -7     
====================================================
- Hits                   8757     8745      -12     
  Misses                    4        4              
  Partials                  3        3              

Files Changed	Coverage Δ
...erimental/autoqasm/converters/return_statements.py	100.00% <100.00%> (ø)
...ket/experimental/autoqasm/transpiler/transpiler.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[yitchen-tim]

Looks great!

[rmshaffer]

Makes sense overall, thanks for doing this! Left a few comments/questions.