[hiopy] add variable attribute for enabling output of this variable

Motivation

Currently we use a ugly heuristic to determine if a variable is added to the output.

Task description

• Add a attribute to the variable (hiopy::enable) which marks a variable for output. (handled in !37 (merged) )
• Remove the heuristic (first dimension needs to be called "time", last dimension "cell")
• Adapt configure.py
• Docu
• adapt tests

Definition of done (Max 3 points)