foucalc: Switch RMS error calculation from Uttley+14 Eq. 14 to Vaughan+03 Eq. 11

* Calculations of the RMS uncertainty are not consistent in the
  literature. There are different pre-factors used in Uttley et al.
  (2014, Eq. 14), Ingram et al. (2019, Eq. 22), and Vaughan et al.
  (2003, Eq. 11).
* Previously, Uttley+14 Eq. 14 was implemented but the uncertainties
  appeared unrealistically small.
* The uncertainties using Vaughan+03 Eq. 11 are larger (although still
  small).