Shall and endpoint table

[philvarner]

…implemented for each conformance class

Related Issue(s): #195

Proposed Changes:

1. change wording of required endpoints for each conformance class from "should" to "shall".

Technically this is a breaking change, but it was understood before that these were required, we just used the wrong word in the spec.

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• ??? This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[cholmes]

Looks good. Do we define somewhere our use of should / shall? Like refer to a document or explain it somewhere?

[philvarner]

We don't have it explicitly stated anywhere, but I think it's widely known enough that we don't need to.