1505 Generalise Parameter Access

[trisyoungs]

This PR simplifies parameter (ExpressionVariable) creation, storage, and handling, moving it all to the ProcedureNode base class rather than requiring override functions in any node type that required to use them.

To handle the automatic prefixing of ExpressionVariables in the parameters_ vector with the node's name (necessitating keeping a record of the original name), a couple of approaches were initially tried and discounted:

1. Modifying the storage of parameters_ from std::vector<std::shared_ptr<ExpressionVariable>> to std::vector<std::pair<std::shared_ptr<ExpressionVariable>,std::string>> with the string component storing the original, base name of the variable for reference - this has the effect of "polluting" a lot of other routines using the parameters, and so was abandoned.
2. Determining the original basename of the variable by searching the string for the last . (if it existed). However this didn't feel particularly robust and required other routines to determine whether or not a prefix existed.

So, the preferred solution in the end was to extend ExpressionVariable to keep track of both it's original name and any applied prefix. This way, impact on the rest of the code is kept to a minimum.

Closes #1505.

[rhinoella]

All looks great, it really cleans up the code nicely.