Remove duplicate declarations for external statements (fix #57)

[reuterbal]

Fortran's external-stmt allows to declare procedure symbols as external separate to their return type declaration. That makes it virtually impossible to determine that this return type declaration refers to a procedure rather than a local variable until the corresponding EXTERNAL statement has been encountered.

Because Fortran allows to represent this also as an attribute in the same declaration, we chose to represent this in Loki as external procedures in all cases. This means, we are replacing

        REAL :: ext_func
        EXTERNAL ext_func

by the equivalent representation

        REAL, EXTERNAL :: ext_func

The frontends already translated external statements to the procedure declaration with the EXTERNAL attribute but also left the original variable declaration in place, thus triggering the problem reported in #57.
In this PR, this behaviour is fixed via a sanitzation processing step that drops relevant symbols from the variable declarations.

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/404/index.html

[codecov]

Codecov Report

Attention: Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.02%. Comparing base (d90b5a7) to head (79e36b0).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
loki/frontend/util.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #404   +/-   ##
=======================================
  Coverage   96.02%   96.02%           
=======================================
  Files         197      197           
  Lines       39023    39046   +23     
=======================================
+ Hits        37472    37495   +23     
  Misses       1551     1551           

Flag	Coverage Δ
lint_rules	96.39% <ø> (ø)
loki	96.02% <96.87%> (+<0.01%)	⬆️

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[mlange05]

Looks good. GTG