Default values in `DispersionField` might be wrong

[teutoburg]

The docstring mentions -0.04 and -0.42 as default values, the code uses -0.0 (yes with a minus) and -0.5. This is in disagreement with what the cited source for the formula mentions those values usually are (section 5.2 of https://ui.adsabs.harvard.edu/abs/2017MNRAS.464..356V).

See also https://ui.adsabs.harvard.edu/abs/2006MNRAS.371.1912B