Updated AMPS model

Updated the model coefficients - now based on a larger dataset from Swarm. The new model version is 1.4

Also:

• Added scripts that use pyAMPS to produce updated versions of the figures from the paper
  (see https://klaundal.w.uib.no/2021/07/21/updated-version-of-amps/)
• Removed LaTeX dependency
• Performance improvement to the featherplot function
• Minor fixes