Lattice and input files for the LCLS accelerators (including the LCLS-II project)

Many of the paths will use the $LCLS_LATTICE environmental variable, which should point to this repository, as in:

export LCLS_LATTICE=/path/to/this/repo

We now use release tags of the form YYYY.MM.DD (full year, short month, short day) according to https://calver.org. This should be more memorable and align better with the physical LCLS machines.