refactor bindings to avoid copies of 1D arrays in evalp

Since, by definition, `xt::pytensor` cannot be reshaped, and since
`py::array` cannot be manipulated in an intermediate step on the C++
side without compromising on it being accepted as an `xt::xexpression`
type afterwards, the only viable option seemed to be to add a Python
layer to the bindings so that the 1D numpy arrays can be reshaped into
2D numpy array views before calling the C++ extension.

As part of this refactoring, the default values for optional parameters
are also removed, and the function parameter types are specified
directly in the Python layer.