rename io to shift_io to avoid a clash with cython io module
add overall scaling to xcamshift using pyPot.scale()/setScale()
xplor_reader now reads restraints in xplor nih style from a string
improve error handling and warning messages
add addRestraints method to xcamshift to match expected xplor idiom
move conversion of xplor shifts into Observed_shift_table

this implies a coupling of Observed_shift_table and
Xplor_reader.Shift_data... Shift_data should be moved to its own module
an default value of the error in the shift is now represented as None
add the ability to replace or update a component
add ability to have an error for each shift

the error is translated into a flat bottomed region for the error
function where no penalty is paid an no forces generated