Add _symbol property to Routine.

[LonelyCat124]

Rather than the RoutineSymbol belonging to a Routine's SymbolTable, we should instead attach it to the Routine as a property (symbol) similar to how we do for References. This symbol should match (or just be ) the parent containers symbol. This therefore depends on #2575 and probably replaces @JulienRemy 's PR #2570 .

This will change a few things inside PSyclone:

• All references to own_routine_symbol will need to use this if they expect a RoutineSymbol
• own_routine_symbol may be no longer needed, and functions will just have a DataSymbol to their own name when required.
• fparser2.py needs to be update to understand what it should construct for Routines.

Some tests/constructs that involve a Routine without a Container above it will have issues with this implementation - if we have these we will need to work on how to fix them.