🚀 Pull Request

Addresses #75. I think we can probably close #16 as well.

Description

This makes it possible to plot cftime's universal datetime object, which was introduced in cftime 1.3.0. The following example now works:

import random

import cftime
import matplotlib.pyplot as plt
import nc_time_axis

dt = [cftime.datetime(2017, 2, day, calendar="noleap") for day in range(1, 28)]
temperatures = [round(random.uniform(0, 12), 3) for _ in range(len(dt))]
plt.plot(dt, temperatures)

We will raise a ValueError if someone tries to plot dates without a specified calendar.

I have taken the liberty to update the example in the README to follow this approach, because it is simpler than using CalendarDateTime. Since nc-time-axis now requires cftime >= 1.5 (#69), this update should be safe.