Correctly going between rail positions

Rewriting the rails positioning part of GDX.ini to correctly go from
start to finish, even if the user entered the numbers the other way
round. And also be more clever with step size and other things.
More clever positioning

- Generate the positions on the rail a bit cleverer, through list
  comprehension. This makes it easy to use in a for loop with
  `enumerate`, so we can drop the counter.
- Write the image size of the raw images into their filename. Makes it
  easier to load them afterwards with numpy ->
  DetectWhichImageHasBestMTF.py or FocusFOVViewer.py
- `pep8`ify
Renamed File to better represent what it's doing
Remove all the cruft, calculate mean and STD

Going step-by-step, I'm removing all the (for now) unnecessary functions
to calculate the power spectral density and MTF. They will come back
from MTF2.py, but for now we don't need them.
The script now loads (raw) images from DevWare, calculates the mean
(exposure) and STD and plots these values. Proceeding here to calculate
the MTF and other niceties.