Add NMODL support for multiple INITIAL blocks

[JCGoran]

Fixes #3588.

The implementation of MergeInitialBlocksVisitor basically does the following:

• check if there is > 1 INITIAL block in the AST. If not, do nothing
• add the found INITIAL blocks to a container, blocks_to_delete
• iterate over all of the statements in each INITIAL block found (in the order of their appearance in the mod file), and add them to a container, statements
• add an additional INITIAL block to the top-level AST, and dump statements in it
• delete blocks_to_delete from the AST
• if passes --nmodl-ast is enabled, write a file <modfile>.<number>.merge_initial_block.mod

Some remarks (for future reference):

• collect_nodes doesn't actually collect Nodes, but rather (a vector of) instances of its parent class, Ast. This means that we can't directly pass it to Program's erase_node. As a workaround, one can use the following idiom (where is_<some_type> is one of the methods here):

auto& blocks = program.get_blocks();
for (const auto& block: blocks) {
   if (block->is_<some_type>()) {
     // logic goes here
  }
}

• the Ast class doesn't appear to implement operator== so we can't compare values at the AST level, but only at the level of NMODL

[github-actions]

✔️ b09e48a -> artifacts URL

[github-actions]

✔️ 1a91380 -> artifacts URL

[azure-pipelines]

✔️ b09e48a -> Azure artifacts URL

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.47%. Comparing base (823fb26) to head (83e5638).
⚠️ Report is 42 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3589      +/-   ##
==========================================
+ Coverage   68.46%   68.47%   +0.01%     
==========================================
  Files         685      686       +1     
  Lines      116721   116754      +33     
==========================================
+ Hits        79914    79949      +35     
+ Misses      36807    36805       -2     

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[azure-pipelines]

✔️ 83e5638 -> Azure artifacts URL

[github-actions]

✔️ 83e5638 -> artifacts URL