Remove case inconsistency for namelist group names and variables

Currently, using a namelist group or variable twice with letters differing in case results in duplicate variables or breaks group inheritance.

Two possible solutions:

• Treat group names and variables as case insensitive, eg. explicit identify variables with different case. Less error prone but only applicable for Fortran namelists
• Keep the letter case as is, leaving the user to take care of correct spelling but allow use of namelist-like file in a case-sensitive environment