User-defined timescale in time_bins()

[willgearty]

It would be great to give more flexibility to the user to which timescale is being used for time_bins() (like axis_geo()). This could include incorporating other timescales through deeptime::getTimeScale() or by letting the user supply their own dataframe. I imagine this would be most useful for the equal-length binning aspect of time_bins().

[LewisAJones]

@ChristopherDavidDean we should try to get this implemented for version 1.1.0. I don't think it will be too difficult tbh.