Created the Chemical_shift.test_read_xeasy system test.

This is for checking the reading of chemical shifts from a 2D XEasy peak list.
Implemented the reading of chemical shifts in the lib.spectrum.xeasy.read_list() function.
Created the Chemical_shift.test_read_nmrview system test.

This, if not obvious from the name, is for checking the reading of chemical shifts from an NMRView
peak list.
Implemented the reading of chemical shifts in the lib.spectrum.nmrview.read_list() function.
Assignments can now contain lowercase letters in Sparky and NMRPipe seriesTab peak lists.