Introduce ASTExternalVariableType

[clinssen]

This is a design idea for a refactoring/cleanup.

ASTExternalVariable now covers several scenarios: fetch a variable from the postsynaptic partner, fetch a variable from the postsynaptic partner at a specific time in the spiking history buffer, and at a specific time in a continuous-time history buffer.

To keep these cases separated, it might be an option to introduce a variable type enum covering each of these cases as a class attribute.

This would help to clean up the following lines of code in the printer:

nestml/pynestml/codegeneration/printers/nest_variable_printer.py

Line 65 in dea633c

# print external variables (such as a variable in the synapse that needs to call the getter method on the postsynaptic partner)