Pre-release (alpha2) of v2.1

The is a second alpha of v2.1.

Some visible new features in v2.1

• you may use "register-sky" recipe instead of "thar" (register-sky requires "SKY" in the recipe_log file, and does not use THAR). This should be useful for the observations taken after the lamp change in June.
• it saves "*.var2d.fits" file which is a 2-d variance map (contributed by Kyle Kaplan).
• The flattening code in "a0v" recipe is improved and should be more usable.
• a new recipe "tell-wvsol" (or wvlsol-tell which is an alias) contributed by Kevin Gullikson. This can be run for a0v stars after the "a0v-" recipes. And it will try to refine the wavelength solution using the telluric model. The results will be saved as ".wave.fits" (for now, the unit is in nm but may be changed).