datetime64 slider text support

[dcherian]

Closes #20

This makes it marginally better:

What do I need to do to make this much, much better ;)

[t-makaro]

I'm not sure how I feel about special casing this, but if I user specifies a value for valfmt, then it should not be overridden.

[TomNicholas]

In xarray I made a special case which uses pandas.to_datetime() to format this as '2000-04-05 00:00:00', but the zeros still get hidden behind the play/pause button:

I think the ideal general solution to this would mean the positioning of the play/pause button needs to be intelligent: it needs to take the length of the text string into account.

[t-makaro]

It might be a good idea to move the default play/pause button location or make the automatic placement more intelligent, but I think that is beyond the scope of this PR.

[t-makaro]

Is pd.to_datetime needed? When using np.datetime64 and valfmt='%s', the timeline slider formats just fine (in my tests). The datetime is displayed the smallest significant value.

import numpy as np
import animatplot as amp
import matplotlib.pyplot as plt

bimonthly_days = np.arange(0, 60)
base_date = np.datetime64('2017-01-01')
dates = base_date + bimonthly_days

t = np.linspace(0, 3, len(dates))
x = np.arange(0, 10, .2)

X, T = np.meshgrid(x, t)
Y = np.sin(X+T)

block = amp.blocks.Line(X, Y)
timeline = amp.Timeline(dates)

anim = amp.Animation([block], timeline)
anim.timeline_slider(valfmt='%s')

plt.show()

Obviously, the play/pause button overlapping is still an issue.

[dcherian]

Made the changes. I'm not sure how to test it though.