`create_kernel` API update

Updating the API of pystencils, mainly `create_kernel`.
`create_kernel` and `create_indexed_kernel` were merged to
`create_kernel`. `create_indexed_kernel` can be still reached,
`create_domain_kernel` is the old `create_kernel`.

A dataclass `CreateKernelConfiguration` was introduced to hold all
necessary values and set them to useful defaults.

Two new Enum classes were introduced: Target and Backend.
This should ensure a proper usage, instead of the string values, which
were misused.