boost.py - Remove default value for r_s + fix docstring + improve function names

[combet]

• The boost factors defined in boost.py uses a default value of r_s of 1000 kpc. While the docstring does say the unit of r_s should match that of the radius vector, it is confusing to see a default value in kpc. I suggest to remove the default value to "force" the user to provide their value of r_s.

• Also the docstring for boost0 in either the NFW or power law parametrisation is not correct (that should "Boost normalisation" and what is currently under boost0 be under Returns)

• Finally, probably need to rename the functions correct_sigma_with_boost*, as it will generally be applied to DeltaSigma or shear rather than Sigma. Also, the NFW boost is based one the NFW DeltaSigma analytical expression, not on the Sigma one