relax SVN archive

Added the new user function select.sn_ratio, to select spins with signal to noise ratio above a specified ration.

The default ratio for signal to noise selection is 10.0. But should probably be 50-100 instead.
THe default of 'all_sn' is True, meaning that all Signal to Noise ratios for the spins needs tpo pass the test.

Added two new systemtests in Spectrum. test_select_sn_ratio_all and test_select_sn_ratio_any.

These test the user function select.sn_ratio.

Added new backend function in pipe_control.spectrum.sn_ratio_selection.

This is to select spins with a signal to noise ratio, higher or lower than the specified ratio.

Added the new user function deselect.sn_ratio, to deselect spins according to their Signal to Noise ratio.

Added function in pipe_control.spectrum.sn_ratio_deselection(), a function to deselect spins according to the Signal to Noise ration.

The function is flexible, since it possible to use different comparison operators.
And the function can be switched, so a selection is made instead.

Added two new systemtest in Spectrum. test_deselect_sn_ratio_all and test_deselect_sn_ratio_any.

These test the user function deselect.sn_ratio, to deselect spins with a signal to noise ratio lower than the specified ratio.

Made the user function spectrum.sn_ratio smaller.

Added float around values in signal_noise_ratio() function.

For the specific analysis of "noe", "relax_disp", and "relax_fit", initialised the sn_ratio parameter structure.

Added the common API Parameter structure 'sn_ratio' in parameter_object.