499 avoid exponential loop handling

[hiker]

Fixes the exponential behaviour with number of non-blocked loops in one Fortran unit. This is done by detecting when searching for a blocked loop in e.g.:

      do 10 i=1, 10
 10     a(i) = i

That the statement with label 10 indicates that there is no matching 10 enddo (or continue) statement coming, and therefore abooting earlier.

It cuts down parse time for a socrates file that was not parsed in 40 minutes(!!) down to 7 seconds.

Problem: I don't know how to test this patch. During development, the existing tests have flagged some issues I had, so the code is definitely used, and would trigger a failed test if it is really broken. But I don't really know how to test if it triggers as expected (since the function itself would return None without this patch as well ... just later)

Suggestions welcome

[codecov]

Codecov Report

❌ Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.21%. Comparing base (a197a85) to head (269153c).

Files with missing lines	Patch %	Lines
src/fparser/two/C99Preprocessor.py	90.90%	1 Missing ⚠️
src/fparser/two/utils.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #500      +/-   ##
==========================================
- Coverage   92.21%   92.21%   -0.01%     
==========================================
  Files          87       87              
  Lines       13832    13856      +24     
==========================================
+ Hits        12755    12777      +22     
- Misses       1077     1079       +2     

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[hiker]

Oops, this was not based on master. Making a new one.