add initial data reader that can be instantiated
add a very primitive xplor chemical shift data reader
add better tests based on alvins data
add a weight for each chemical shift
add a class field

note the class can't currently be set...
use a named tuple for the results
add tests that empty and multi atom selections raise an exception

selections should only contain one atom...
add better identification of lines that cause errors
add ability to read shifts with out an error (set to default)

not the default error is currently set to the arbitary value of 0.1
add error  handling for fields with too many or too few values
add error handling for bad float data and test
rename xplor reader data source access method to make purpose clearer
refactor getting atrom selection an raising error into methods
refactor processing fields in assign statement into separate methods
remove commented out unused code
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