Add to docs, mend get_active_link_connecting...

Mended get_active_link_connecting_node_pair.

Modified docstring for create_node_array_zeros so it’s clearer how to
get your data onto the grid fields.

Added placeholder methods for .at_node & [‘node’][…] to the manual
index.

Created a short document for the readthedocs on the woes of Python on
Linux, after some negative experiences this morning.