Integration in vperp in collisions

[EmilyBourne]

The collision operator takes an argument intdmu:

gyselalibxx/src/collisions/collision_operator.hpp

Line 36 in f9a80ac

* @param[in] coeff_intdmu The quadrature coefficients to integrate on the grid of magnetic moments.

However in the Gysela (Fortran) code it is not coeff_intdmu that is provided but rather coeff_intdvperp with:
$d v_\perp = d \mu \sqrt{2 \mu}$

As seen here:
https://gitlab.maisondelasimulation.fr/gysela-developpers/gysela/-/blob/master/src/mesh_old/geometry.F90?ref_type=heads#L427-430

This change should be brought over to the C++ code