Add AmberTools conda recipe

I've created a conda recipe (with @kyleabeauchamp's help) for packaging up `AmberTools` 13.

The package actually compiles, but the `test` phase is hindered by the necessity for the `AMBERHOME` environment variable to point to the base directory in which the `bin`, `lib`, and `dat` files are placed.  While this would be `$PREFIX` at `conda build` time, neither @kyleabeauchamp nor I could figure out what directory that would be for the user at install time, or how to have the environment variable set for the user.

I suspect we will have to automatically patch all occurrences of `AMBERHOME` in the AmberTools source tree with whatever environment variable will be used at runtime after conda package installation, and make sure this environment variable is also correctly set at build time.