updated the version number to 2.3.1 after adding emittance calculations to the SP_relativity

ess/__init__.py

 __all__ = ["fieldmap", "installed", "lib_tw", "SP_Relativity", "TraceWin" "TTF"]
-__version__ = '2.3'
+__version__ = '2.3.1'
 from . import TraceWin
 from . import installed
 from . import lib_tw