Fix for the lib.dispersion.cr72 module for early Python versions.

For Python 2.5 and earlier, the math.acosh() function does not exist.  Therefore the numpy
equivalents are now being used.
Loosened the checks for the Relax_disp.test_hansen_cpmg_data_to_ns_cpmg_2site_star_full system test.

This is to allow the test to pass on 32-bit Linux systems.