added VSS model to dissociation integral and cross-section functions, replaced rig_sphere with model parameter, merged raw VSS and RS versions of vv and vt functions
added VSS model and model parameter, replaced _rigid_sphere_ in function names with _integral_
merged raw functions (now 2 instead of 4), replaced rig_sphere parameter with model parameter
comments updated
replaced rig_sphere with model parameter
new docs
new docs
new docs
new docs
v0.3