Analyser/Generator cleaning up

[agarny]

Work on issue #1243 in PR #1247 has resulted in some inconsistencies which we need to fix.

Indeed, in PR #1247, we replaced instances like const AnalyserVariablePtr &variable with const AnalyserVariablePtr &analyserVariable, but we haven't done it for const AnalyserModelPtr &model, const AnalyserEquationPtr &equation, const AnalyserEquationAstPtr &equationAst, const AnalyserExternalVariablePtr &externalVariable, etc. and... also with their internal variants (e.g., const AnalyserInternalVariablePtr &variable).

So, this issue is ONLY about "cleaning up" our Analyser and Generator codebase. Nothing more nothing less.