Julia: refactor the auxiliary variable/series files

— rename the files with camel case
— encapsulate the functions with modules
— change the signature of function <MODFILE>_dynamic_set_series!, by removing
  the output argument and appending the exclamation mark, since this function
  modifies one of its arguments, and for symmetry with the static version

Ref. DynareJulia/Dynare.jl#1