[Therion] Metapost area block

[Andrew Atkinson]

After a meeting the UK Cave Surveying Group had in November, some
thought the the block did not look that good, and could be improved,
with different sided block. Work started and I have eventually got round
to finishing it. Based on the AUT version (which is based on others by
the looks of it.) I have made it so that there are variables at the
start so people can adjust it to their personal preference with nearly
no knowledge of MetaPost (not that I have much!)

https://therion.speleo.sk/wiki/metapost#customisable_area_blocks_with_different_number_of_sides

The loop for drawing different sided blocks could be improved, this is
the quick and dirty play.

Some things I could really do with help improving. To get rid of the
transparency of other blocks below, I have had to call thclean three
times, there has to be a better way.

Also blocks at the edge, there tends to be too much space, I have an
infinite loop, which works, sort of, but tends to leave spaces, a better
solution would be good. Blocks tend to pile up near walls, but that
would need to be able to differentiate between the lines that make up
the border, can that be done?

I have also added a fixed rotation, then some variability around that,
to try to simulate bedding blocks. Currently uniformdeviate is used,
MetaPost seems to have normaldeviate, which may be better for this, but
when trying it in Therion, it appears not to be a recognised command?

This also leads to the question, can any of these settings be accesses
from the th2 file, maybe as an option rather than by changing the MetaPost?

For people to play, I've placed the test files at
http://wotcc.org.uk/__Survey/BoulderTestArea.zip

sorry lots of questions, thanks in advance

Andrew