Move new credit class SHACL constraints to generic credit-class.ttl

[blushi]

As part of the creation of C04 and KSH01, some new metadata fields have been added: #68, #66
The constraints defined for them have been added to class specific files but we realized that most of them relate to the same fields.
So we should refactor those constraints to be in our generic credit-class.ttl instead (and make them optional since they might not be in the other credit classes metadata).

We can work on that once #66 and #68 are merged.