Add array-API support

[OleBialas]

I have added support for array-api as described in gh-38
Now, we can use any kind of array supported by the standard.
For example, one could use CUpy and compute TRFs with GPU acceleration.
Thanks to the magic of array-api-compat this does not require any breaking API changes.

I did make some changes to private functions though and adapted the tests accordingly.
I also changed a few outdated docstrings and started adding type hints.

[powerfulbean]

Thank you @OleBialas for this huge updating and work!

[OleBialas]

My pleasure! Maybe we could do some benchmarks to measure training with and without GPU and add a section to the docs.
By the way, I'm just returning from a retreat organized by the Max Planck Research School where several people told me they are using mTRFpy :)

[powerfulbean]

My pleasure! Maybe we could do some benchmarks to measure training with and without GPU and add a section to the docs. By the way, I'm just returning from a retreat organized by the Max Planck Research School where several people told me they are using mTRFpy :)

Thanks Ole, glad to hear this! It sounds good to me that we do some benchmarks. I was pretty busy during the past month, but I will start adding new functions following the array-API standard this week.