Add error when plotting cftime

[cvanelteren]

Plotting on non-gregorian time scales raises an error. Matplotlib by default does not allow for this. Luckily there is a fix. By installing nc-time-axis some formatters are registered that allows for this. We can either implement this natively or add an install requirement

Currently I just implemented an error when one is plotting in a non-gregorian calendar (issue raised in #288).

[cvanelteren]

I am open for bundling the package directly. The source is just a simple formatters that are registered.

[codecov]

Codecov Report

Attention: Patch coverage is 45.45455% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ultraplot/internals/inputs.py	45.45%	5 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[beckermr]

The package seems well maintained, so let's depend on it. This PR needs a test.

[cvanelteren]

Will add some integration. As @pgf pointed out in #288 we need to format the formatters to make them behave similarly to the default datetime for proper styling. Hopefully this is straightfoward.