Prepare for 1.21 release (NanoComp#1786)

Prepare for 1.20 release (NanoComp#1723)

* Prepare for 1.20 release

* remove nightly built Conda package from the installation page

* update description for subpixel smoothing of MaterialGrid

* bump 1.20.0 date, update links

* whoops

* version bump

Co-authored-by: Steven G. Johnson <stevenj@alum.mit.edu>

version bump

more NEWS links

bump to 1.17.1

more copyright bumps

1.16.1 bump

meep 1.16.0

meep 1.15 bump

nbviewer links