option to install matplolib backends

[chrisjsewell]

See https://matplotlib.org/3.1.1/tutorials/introductory/usage.html#what-is-a-backend

Previously in ansible-role-aiida, for debian:

• python3-gi
• python3-gi-cairo
• gir1.2-gtk-3.0

for redhat:

• gtk3
• python3-gobject

For debian, according to https://askubuntu.com/a/1166300, you should be able to just install with the generic python3-, then copy the so files for the required python version, e.g.

cp /usr/lib/python3/dist-packages/gi/_gi_cairo.cpython-36m-x86_64-linux-gnu.so /usr/lib/python3/dist-packages/gi/_gi_cairo.cpython-37m-x86_64-linux-gnu.so
cp /usr/lib/python3/dist-packages/gi/_gi.cpython-36m-x86_64-linux-gnu.so /usr/lib/python3/dist-packages/gi/_gi.cpython-37m-x86_64-linux-gnu.so
cp /usr/lib/python3/dist-packages/cairo/_cairo.cpython-36m-x86_64-linux-gnu.so /usr/lib/python3/dist-packages/cairo/_cairo.cpython-37m-x86_64-linux-gnu.so

(I tested this and it looks to work; import gi, cairo fails before copy and works after)

[chrisjsewell]

as with #5 this aslso ideally requires testing

[chrisjsewell]

probably use python_mpl_gtk3 option