Support for different safety level of type casting for numpy array input

[wenjuno]

Numpy array astype accepts casting parameter to control what kind of data casting may occur. It can be used to catch unintended and/or undesirable casting (e.g. truncating cast of float to int). Nanobind uses the default "unsafe". It would be great if we can add control on the kind of casting other than just no-convert and unsafe-convert.