Cython constructor for sample sets

It would be useful to be able to construct sample sets from c++ containers using [Cython](https://cython.org/). Something like
```
from dimod cimport sampleset_from_vectors

cdef Py_ssize_t num_variables = 10
cdef Py_ssize_t num_samples = 5

cdef vector[int] samples(num_variables*num_samples)
cdef vector[float] energies(num_samples)

sampleset = sampleset_from_vectors(samples, 'SPIN', energies)
```