A first trial at passing the "curve_type" to the C code.
However, something is wrong, potentially in the C code (see
compilation warning below) as relax now complains that
"an integer is required" for assignment to "curve_type":
====
maths_fns/relax_fit.c: In function 'setup':
maths_fns/relax_fit.c:66: warning: assignment makes integer from pointer without a cast
====
