(Closes #320) nonstandard stop

[hiker]

Fixes #320. Adds support for extended stop codes to Fortran 2003, and adds full 2008 support for (even more) flexible expressions.

I hope I did the 2008 part correctly. I esp. wasn't certain how to check if the expressions are constant :(

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.14%. Comparing base (55db863) to head (17a1078).
Report is 13 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #451   +/-   ##
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  Coverage   92.13%   92.14%           
=======================================
  Files          85       86    +1     
  Lines       13824    13834   +10     
=======================================
+ Hits        12737    12747   +10     
  Misses       1087     1087           

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[arporter]

Thanks very much for this Joerg. Generally all looks good but I'd like a couple more tests to show that parsing STOP statements works in the context of a full parse and that the extension can be switched off. I'd also like some tests to show that the F2003 forms are still accepted by the F2008 parser.
Updated docs build fine (modulo the hundreds of existing warnings) and look good.

[hiker]

Ready for next review.

[arporter]

Thanks for extending those tests Joerg. All good now. I've tested it with the PSyclone test suite and all is fine.
There are couple of bits of tidying needed but as that's just an import statement I'll do that.