Add dpctl dtype objects

[ndgrigorian]

This PR proposes the addition of _dtypes.py, which implements dtype-related classes for dpctl, as well as a broadening of the permitted inputs as dpctl kernel arguments

Previously, complex scalars and float16 could not be passed to kernels, as ctypes lacks either of these constructs. Additionally, the kernel submission functions required ctypes arguments, rather than generic Python scalars or even NumPy scalars.

This PR proposes the addition of a DpctlScalar base class which defines scalars (using NumPy) and dpctl.dtype, which carries dtype metadata, as well as the expansion of permitted kernel arguments to include NumPy scalars and Python scalars

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/2335/index.html

[coveralls]

coverage: 75.013% (+0.4%) from 74.579% — add-dpctl-dtypes into master