when to evaluate arguments and when to meta-unroll.
-use it in Product to determine when to eval args. not yet used
to determine when to unroll. for now, not used anywhere else but
that'll follow.
-fix badness of my last commit
and long double.
-define scalar-multiple operators only for the current Scalar type;
thanks to Gael for expaining how to make the compiler understand
when automatic casting is needed.
-take ScalarMultiple take only 1 template param, again.
We lose some flexibility especially when dealing with complex numbers,
but we gain a lot of extensibility to new scalar types.
