Fastkde

* Bug fix for `fastkde.conditional` (48)
* Fixes wheel building and upload to pypi
* Support for numpy >1.26.4, including numpy >2.0

The [upgrade to numpy 2.0 causes an API compatibility issue](https://numpy.org/doc/stable/dev/depending_on_numpy.html#depending-on-numpy) (thank you AdamOrmondroyd for the initial fix!). Supporting numpy v1 and v2 simultaneously makes it difficult to support older versions of python; support was dropped for python versions less than 3.9 and this is now enforced as a requirement in pyproject.toml.

If python versions less than 3.9 are required, I suggest forcing installation of an older fastkde version, e.g.: pip3 install fastkde<2.1.1; the functionality of fastkde remains the same in v2.0.1 and v2.1.1. Note though that the bug identified in `fastkde.conditional` will exist in these earlier versions; this bug is not an issue if the `xarray` functionality is not used.

This release adds support for numpy >1.26.4, including numpy >2.0

The [upgrade to numpy 2.0 causes an API compatibility issue](https://numpy.org/doc/stable/dev/depending_on_numpy.html#depending-on-numpy) (thank you AdamOrmondroyd for the initial fix!). Supporting numpy v1 and v2 simultaneously makes it difficult to support older versions of python; support was dropped for python versions less than 3.9 and this is now enforced as a requirement in pyproject.toml.

If python versions less than 3.9 are required, I suggest forcing installation of an older fastkde version, e.g.: pip3 install fastkde<2.1.1; the functionality of fastkde remains the same in v2.0.1 and v2.1.1.

(this release is identical to 2.1.0, except that the pip/wheel building process is fixed)

This release adds support for numpy >1.26.4, including numpy >2.0

The [upgrade to numpy 2.0 causes an API compatibility issue](https://numpy.org/doc/stable/dev/depending_on_numpy.html#depending-on-numpy) (thank you AdamOrmondroyd for the initial fix!). Supporting numpy v1 and v2 simultaneously makes it difficult to support older versions of python; support was dropped for python versions less than 3.9 and this is now enforced as a requirement in pyproject.toml.

If python versions less than 3.9 are required, I suggest forcing installation of an older fastkde version, e.g.: `pip3 install fastkde<2.1.0`; the functionality of fastkde remains the same in v2.0.1 and v2.1.0.

Bugfix release - addresses issue with the fastkde/xarray interface and 1D data

* return xarray objects from `fastkde.pdf` and `fastkde.conditional` by default
* add more examples
* simplify and clean up API
* add more comprehensive testing