[Enhancement]: Time to decimal time functionality

[pochedls]

Is your feature request related to a problem?

To regress on time (e.g., to calculate a linear trend), the user needs a time axis in units of decimal year (or something similar), but xcdat defaults to cftime objects. It would be helpful to have a convenience function that could produce an array of decimal time values.

Describe the solution you'd like

An ideal solution would enable you to simply convert a standard cftime axis to decimal time (e.g., xc.to_decimal_time(time)).

Describe alternatives you've considered

No response

Additional context

No response

[pochedls]

I created this issue in part to note that this PR may solve this.

Although it appears we don't have an issue for this feature request, this functionality has long been desired by xcdat users.