Create new tangent type for MajoranaReg?

[jlbosse]

In order to get backward mode AD working I had to add the function :(*)(number::Real, reg::MajoranaReg) which is only a sensible operation if reg is a (co-)tangent register and not sensible if it is the primal register. This opens up potential incosistencies because we can do
expect(op, a * reg) but this doesn't satisfy expect(op, a * reg) == a * expect(op, reg).

One possible solution here is to add a TangentMajoranaReg type and only allow multiplication
with a scalar for that one.