fieldmap already documented this way

bumped version to 2.5.0

use git tag to set version

which we anyway don't use

- we stop supporting python 3.5

a lot of blank except: were replaced,
maybe some of them incorrectly

ess.lib_tw is broken due to removing star imports

Update SP_Relativity.py with functions emit_geo and emit_norm which return the geometrical and normalized emittances