skan should support float32 images

[jni]

In #234 and #235, @ns-rse noticed that passing float32 images doesn't work in newer versions of skan (which pass the image unmodified rather than a float64 copy. That's because the numba jitclass declaration has an array of fixed dtype of float64[:]:

skan/src/skan/csr.py

Line 162 in 3d5da88

('node_properties', numba.float64[:]),

so passing float32 data to it causes an error. But! It would be good to be able to support float32 data, not least because it uses half the RAM, for limited loss in precision. So we should figure out a way to do that, maybe switching out two classes, NBGraph and NBGraph32, for example, depending on the input data.