Add Scalar template argument to UnalignedEquivalent struct.
Add Scalar template argument to UnalignedEquivalent struct.

* It allows creating unaligned equivalent of Eigen matrix using a
  different scalar type. This can be useful when matrix types use
  single precision floats, whereas Numpy matrix types use double
  precision floats by default.
* Default template argument value is Matrix D scalar type to preserve
  backward-compatibility.

jcarpent/eigenpy,jcarpent/eigenpy,jcarpent/eigenpy,stack-of-tasks/eigenpy,stack-of-tasks/eigenpy