fix(csr): Set type of node_props to float64

`dtype` of `node_props` passed from `csr_to_nbgraph()` to `NBGraph` was `float32`. This sets the type to explicitly be
`np.float64` which matches the expectation of `csr_spec_float` passed to `numba.experimental.jitclass()`.

src/skan/csr.py

@@ -207,7 +207,7 @@ def csr_to_nbgraph(csr, node_props=None):
             csr.indices,
             csr.data,
             np.array(csr.shape, dtype=np.int32),
-            node_props,
+            node_props.astype(np.float64),
             )