se graph change and speeding/tidying up

Have changed sentence graph (and graph actions inc. TextRank algorithm)
from double directed (both forwards and backwards edges) to undirected
(edges without direction). Results unchanged. Processing speed a tiny
bit slower for current essays, but a bit faster for larger texts.
Now not checking textbook index during removal of stop words because not
achieving anything and taking up relatively a lot of processing time.
H810_TMA01_textbook_seale_index.py removed from EssayAnalyser directory.