Support cftime objects for string templating

[JamesVarndell]

Is your feature request related to a problem? Please describe.

Some netCDF datasets store datetime information using the cftime library. Inside earthkit-plots, we use pandas to convert datetimes to strings - but pandas is not fully compatible with cftime, causing some failure when attempting to plot data with cftime-encoded times.

Describe the solution you'd like

We should be able to detect when datetime information is in a cftime format and convert it properly to a datetime.

Describe alternatives you've considered

No response

Additional context

Related to #35

Organisation

No response