Fix a parameter in Eddington CLI

[saroad2]

When fitting data with a fitting function, sometimes we want to fix one of the parameters and see what happens. For example, when we want to fit data, we often use the general parabolic function:

But, sometimes we want to consider b=0, which means we want to fit:

Therefore, one should be able to fix a parameter to specific value. Optional syntax:

edddington parabolic --fix 1 0 --csv-data /path/to/csv

aka, fix value in index 1 (indices starting from 0) to be 0.