shape-typing in scipy.interpolate.BSplines

[jorenham]

The coefficient array c is at least 1d. Its shape affects the output shape when called. E.g. when c is 1d and the BSpline instance is called with a scalar, the return type will be a 0d, and when c is 2d, the return type will be 1d.

By making BSpline generic for the shape-type of c (i.e. as _ShapeT_co: onp.AtLeast1D = onp.AtLeast1D), it becomes possible to annotate the shape-type of the __call__ return type, or at least for some of the common combinations of _ShapeT_co and x (the input to __call__).