Fix namelist r552 matching error (closes #165)

[rupertford]

The implementation of namelist-stmt r552 in Fortran2003 (Namelist_Stmt) contained asserts in two places rather than returning None (which specifies no match). This PR fixes this error, tidies the code, tightens the implementation and adds tests that fully cover the code in the class.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (fbcc2c6) 91.75% compared to head (a0d8253) 91.76%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #415   +/-   ##
=======================================
  Coverage   91.75%   91.76%           
=======================================
  Files          37       37           
  Lines       13384    13387    +3     
=======================================
+ Hits        12281    12284    +3     
  Misses       1103     1103           

Impacted Files	Coverage Δ
src/fparser/two/Fortran2003.py	94.41% <100.00%> (+<0.01%)	⬆️

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[rupertford]

Ready for first review from @arporter or @sergisiso

[arporter]

Looks good. Changes are pylint clean.
Coverage is all good.
This is a bug fix so no documentation update required.
Issue number is not mentioned anywhere.
Will proceed to merge.