plot_hdi: add exception if x is type np.datetime64 and smooth=True

[drbenvincent]

In #1968 I pointed out that az.plot_hdi failed if x was of type datetime when smooth=True. This PR adds an exception to hopefully make this easier for a user to work out what is going wrong.

FYI this is my first PR to arviz, so apologies if I've missed some steps.

[codecov]

Codecov Report

Merging #2016 (eb05905) into main (b2b9cbc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2016   +/-   ##
=======================================
  Coverage   90.74%   90.75%           
=======================================
  Files         117      117           
  Lines       12453    12455    +2     
=======================================
+ Hits        11301    11303    +2     
  Misses       1152     1152           

Impacted Files	Coverage Δ
arviz/plots/hdiplot.py	92.72% <100.00%> (+0.27%)	⬆️

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[OriolAbril]

Looks great, thanks! Can you add a test similar to https://github.com/arviz-devs/arviz/blob/main/arviz/tests/base_tests/test_plots_matplotlib.py#L1188 to check this error is thrown when datetime data is passed (and we therefore don't remove it unkowingly in the future)