spherical expansion coefficients can be freed after the computation of the spherical invariants

[agoscinski]

By freeing the spherical expansion coefficients one can get a decrease of 8-15% (depending on nmax,lmax) in the computation of the power spectrum features. Its not really a significant part (the dense features are the memory expensive part and this is not included in the results below), but it just requires to add two lines of code. I ll make later a PR.

memory usage of this was measured

soap.transform(frames)

for one species (envs 17743, nmax,lmax=6)
Memory usage 187.4 MiB (freeing sph exp)
Memory usage 211.1 MiB
for 4 species (envs 17743, nmax,lmax=6)
Memory usage 497.1 MiB (freeing sph exp)
Memory usage 539.7 MiB