implement a mecanism to activate/deactivate generation of external variables with 'input' prefix

[lenaRB]

@lenaRB: as suggested by you to ease testing, implement a mecanism to activate/deactivate the way external variables are translated in Modelica: either as "classical" variables or as variables with prefix input

as a reminder, please refer to ticket lenaRB/crml-compiler#87 to study what are the pros and cons of translating external variables as inputs or as "classical" variables.

[lenaRB]

• we add input in front of all the variables at the moment, need to have a prefix to turn this on/off