Use "nearest" interpolation for level() plots by default

[mgeier]

Fixes #170.

The plots still don't look great, but at least this restores the previous state.

Rendered: https://sfs-python--172.org.readthedocs.build/en/172/sfs.td.wfs.html

Apparently, the default was changed to nearest in Matplotlib 2.0:

https://matplotlib.org/3.3.3/users/dflt_style_changes.html#interpolation

... and then changed again in 3.2.0 (which is probably the cause for #170):

https://matplotlib.org/3.3.3/api/prev_api_changes/api_changes_3.2.0.html#default-image-interpolation