extinction: update the Vergely implementation to use cached data and interp

[SteveOv]

Update to follow the same pattern as used with the Gontcharov map.

I have the ~8 GB fits files downloaded. Will need a way of loading parsing these for the (X,Y,Z,Exti) values.

Previous work has found that the X,Y,Z are Galactic coordinates, centred on the Sun. Positive X towards Galactic centre & positive Z towards Galactic North. The Exti values are in units of nmag/pc at a wavelength of 550 nm.

Will hold off using this map until the changes are made.

[SteveOv]

May be very difficult. I have 64 GB ram, however that's not enough to load the .dat.txt file into a numpy array.

Can just about open the table in hdul[1] from the .dat.fits, although it takes minutes to do so. It's in an awkward format; millions of rows, each with a tuple of (X, Z, Y, Exti). This could be challenging when I want to split into (X, Y, Z) coords and (Exti) data.