units problem
clarify meaning of effective diameter
Fixed typo install instructions pip install startlink
Merge pull request #609 from connolly/master

Fixed tiny typo in install instructions
Regularize the _H header guards and include GalSim_ as a univeral prefix.
Add missing return statement to ChromaticSum.drawImage
clarify demo5 PSF shifts
Merge pull request #605 from GalSim-developers/#604

#604 Change comparisons with None to use `is` or `is not` rather than `==` or `!=`
Merge branch 'master' into #603