Fixes for gaps in the ELM visitor

[JPercival]

• Added architecture tests to ensure ElmVisitor followed the expected conventions
• Added random ELM graph generation to ensure all nodes are visited
• Renamed ElmBaseVisitor -> BaseElmVisitor to match conventions for abstract classes
• Removed code that was pass through to super class
• Fixed traversal bugs to ensure operands and arguments are visited correctly

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[codecov[bot]]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (cf7c30a) 62.64% compared to head (5fd11a7) 62.70%.

Files	Patch %	Lines
...k/cql/elm/requirements/ElmRequirementsVisitor.java	84.90%	4 Missing and 4 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1308 +/- ## ============================================ + Coverage 62.64% 62.70% +0.05% + Complexity 6158 6079 -79 ============================================ Files 466 466 Lines 25424 25308 -116 Branches 4703 4711 +8 ============================================ - Hits 15928 15869 -59 + Misses 7348 7288 -60 - Partials 2148 2151 +3 ```

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