added setter method for data in braindata, made vertexdata data size error more verbose
added setter method for data in braindata, made vertexdata data size error more verbose

CVML/pycortex,smerdis/pycortex,smerdis/pycortex,CVML/pycortex,gallantlab/pycortex,gallantlab/pycortex,smerdis/pycortex,CVML/pycortex,gallantlab/pycortex,CVML/pycortex,gallantlab/pycortex,gallantlab/pycortex,smerdis/pycortex,CVML/pycortex,smerdis/pycortex