Fix cosmology_get_scale_factor() when a_end != 1

In cosmology.c there's a function cosmology_get_scale_factor() to get the scale factor given the age of the universe. It gives wrong answers if a_end is not 1. In the call to interp_table() I think the last parameter should be the age of the universe at a_end rather than the present day.

This function is only used in output_list.c so this bug only affects runs with an output list specified in terms of time and a_end != 1.