enh: Allow to hide model parameters via key names

[paulmueller]

If a model in nanite contains helper parameters (e.g. lmfit constraints), the developer should be able to hide it. The best way of defining such a parameter is by prefixing it with an underscore.

• update the nanite docs with this information (dec3025b4884fe8c29cc9d7802df1ba24bfdb72f)
• write a test function
• add logic to hide parameters prefixed with "_" from the parameter view ( 55ad5b2 )