WIP Refactor discrete-post-option creation

[schillic]

Closes #379.

Now that #464 got merged, these options still need to be revised. So I keep this PR open as a reference, but we should start a new approach.

[mforets]

This is a valid approach but i have hesitated because removing the function call from Options, we loose:

• checking that the given option has the good type (merge doesn't know)
• the debug options (verbose) in check_aliases_and_add_default_value!
• handling aliases

[schillic]

True, the "real" solution is to have #255.
Both approaches are not perfect. The current implementation uses functions that are not supposed to be used that way. So it is a hack which also does not work.

handling aliases

Currently we do not have aliases for those options.