Issue labeling convention

In order to encourage new contributions, we should use the following labels to distinguish between the level of expertise required, and the amount of effort required (which are not necessarily the same). We have settled on the following labels:

• package-novice - accessible to someone who has no knowledge of astropy internals and can be done without this knowledge.
• package-intermediate - accessible to someone who has at least good experience in using the package and understands the issue well.
• package-expert - accessible only to people intimately familiar with the sub-package/functionality.
• effort-low - a few hours at most
• effort-medium - a few days at most
• effort-high - longer term

The effort should be indicated for the package-... level specified (so the effort may be lower for a more experienced developer). We also use the following tags:

• domain-specialist - indicates that knowledge of the specific domain (e.g. cosmology, coordinate transformation) is needed.

Packages that use the above convention:

• astropy core
• astropy tutorials
• photutils
• ccdproc