Resolving bugs with build and clone processes

[SamuelDegelia-NOAA]

This PR makes a few minor changes to resolve bugs associated with building and cloning, including:

1. Resolving CMake failed on Orion/Hercules #144 by updating the documentation to note the need to run module load git-lfs before cloning on Orion.
2. Resolving FYI, conda: command not found when building under the EVA python environment #135 by adding module purge to the beginning of build.sh. Previously, if you had the eva modules loaded, build.sh would fail. This quick change prevents that error by forcing you to unload all modules before building.
3. Modifying link_mpasjedi_expr.sh by adding the ${BUMPLOC} directory name to prevent a linking error during build on Orion. Previously, the linking failed on due to the link directory not being specified which lead to cannot overwrite directory error. This error did not occur on Hera.

Note that there is still an ongoing bug with building on Orion (#146). Restarting the build resolves that issue, but I am still actively investigating the root cause.

[guoqing-noaa]

LGTM.

Thanks for fixing these bugs and addressing my picky comments.