Switched the optimisation algorithm in test_suite/system_tests/scripts/relax_fit.py.
This script, used by the Relax_fit.test_curve_fitting_height and Relax_fit.test_curve_fitting_volume
system tests, now uses the BFGS optimisation. This is to demonstrate that the exponential curve
gradient function dfunc() is implemented correctly and that more advanced optimisation algorithms
can be used (excluding those that require the full Hessian d2func() function).
