Follow-up from "Resolve "Add missing ENABLE_AMR""

The following discussion from !347 (merged) should be addressed:

• @snuverink_j started a discussion: (+8 comments)

  @frey_m : why do have Options::amr only if ENABLE_AMR=TRUE ?

  Would it not be easier to have this bool always available and set it to false if ENABLE_AMR is not defined and true by default otherwise?

  Also you don't actually need to expose the option (those in BasicActions/Option.h) to the parser, I was merely talking
  about the internal variable.

  Answer by @gsell:

  @snuverink_j yes, maybe this is the right way to go. Internal variables for options/features should be available but not exposed to the parser. This simplifies the code and we can handle this (by throwing an exception) in the parser.