work around pytest issue writing bytecode files on read-only file systems, as suggested by  cslocum
minor: add sanity check against accidentaly negative FOV size
Minor: convert an error case for MultiHexagonAperture into a proper Exception
Fix unit test for ThinLens
Merge branch 'master' of github.com:mperrin/poppy
local changes plus douglase resize optics commit
Merge branch 'master' of github.com:mperrin/poppy