docs: add note on increasing work_mem

[ltalirz]

fix #2859

• add note on increasing work_mem
• restructure troubleshooting section a bit

[kjappelbaum]

After removing the circus references, I would maybe add a hint that can be useful for debugging. If one sets log_temp_files = 0 in /etc/postgresql/10/main/postgresql.confone can check if the writing of temporary files is indeed an issue. Also I would probably recommend querying for the message table size and then adjusting thework_mem` setting based on that as e.g. for me and Daniele the table size was different.

[ltalirz]

thanks for the suggestions - I've tried to incorporate them
in order not to overcomplicate things I don't include instructions for tuning the value depending on the size of the table but I think it contains everything needed to get started debugging

[kjappelbaum]

Thanks, Leo!

[ltalirz]

@sphuber do you have any idea why the tests failed? Seems to be exactly the same error as in #2420

It was thought that this could be prevented by adding the
--no-cache-dir argument to pip here: #2799
but perhaps that was fortuitous?

I also tried resetting the travis cache...

[sphuber]

It does seem to be the same error, and indeed the --no-cache-dir did consistently fix the problem last time. Maybe now there is an incompatibility even when installing everything from scratch, i.e. the version of pymatgen that we tell to install, will automatically install a version of numpy and scipy that are incompatible.

[ltalirz]

I'm going to merge this now... will hopefully fix this travis problem at some point as well.