sgext_image: `from_pyarray(input_array)` should raise error if `dtype` of input array is incorrect

[phcerdan]

sgext internal wrapped images are either binary (unsigned char, dtype=uint8) sgext.itk.IUC3P or float (dtype=float32) sgext.itk.IF3P.

sgext.itk.IUC3P().from_pyarray(input_array) should raise error if input_array dtype is not uint8.

Raised here: #60