Resolve error[E0277]: cannot multiply <P as ModelParameters>::BaseField by <T as AffineCurve>::BaseField`

[drskalman]

SWU_Params is defined as SWModelParameters
MapToCurve depends on AffineCurve

I need to generate the AffineCurve which MapToCurves dependes on out of SWU_param. I might not
need to actually generate the curve but I need the type at least. I don't need to generate the curve,
because Daira's code doesn't require curve operation and it is all in term of elementary algebaric operation
or FiniteField operation.

At least one way, that ArkWork does it is through GroupAffine. AffineCurve has been implemented for GroupAffine<P: SWModelParameters>

impl<P: Parameters> AffineCurve for GroupAffine<P> {

so it makes sense to talk about GroupAffine

as AffineCurve but GroupAffine is a struct. I'd have prefered something like

trait <P: SWModelParameters> SWAffineCurve<P> : AffineCurve

then I could have had

impl <P: SWAffineCurve> MapToCurve<SWAffineCurve<P> as AffineCurve> for SW_hasher<{

Maybe still I can do that with GroupAffine

because struct can be regarded as trait