Add option to load constant parameters from a .par file

[tgeijten]

This is useful when you want to use the result of a previous optimization without further optimization. Options include:

• locked_init_file
• static_init_file
• const_init_file

[tgeijten]

Alternatively, parameters should also be fixed if an entry in a .par file either:

• only has a single value
• has a STDEV of 0