Generalize the templates and input keys.

Currently the `tap` and `xcone` service types assume the input `dict` has keys `ra`, `dec` and `radius`, and it substitutes them in that order into the 3 `{}`s in the `adql` and `access_url` templates.  

Instead, the input `dict` should be able to have arbitrary keys, which would be substituted into the templates with explicit key names (e.g., `{ra}`, `{dec}` and `{radius}`.