Load 'morphology' symbol during registration

[alkino]

This way if someone define again this symbol (in hoc file for example), neuron will use the right one during allocation of the module.

This is how it is currently done in nocmodl.

This behaviour has been spotted with nmodl benchmark.

[codecov]

Codecov Report

Attention: Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.

Project coverage is 86.73%. Comparing base (c5052b9) to head (9a6c7c7).

Files	Patch %	Lines
src/codegen/codegen_neuron_cpp_visitor.cpp	28.57%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1393      +/-   ##
==========================================
- Coverage   86.75%   86.73%   -0.02%     
==========================================
  Files         180      180              
  Lines       13669    13674       +5     
==========================================
+ Hits        11858    11860       +2     
- Misses       1811     1814       +3     

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[1uc]

Usually, we can use get_variable_name("morphology_sym") to get the string we need to print. However, that requires that it's a "known" global variable. Which because it's type is not double is a little tricky.

There's also a risk of clashing with a GLOBAL variable called morphology_sym. Easily avoided by calling it _morphology_sym (since user defined variables cant' start with _).

I think it solves a real problem we're facing. Hence, I'd be okay merging it.