change owlgen to use default_prefix for namespace

[cmungall]

Fixes bug where owlgen placed all classes, properties into meta namespace.
The fix incvolves changing how definition_uri is populated while schema loading.
Fixes #163


[hsolbrig]

A couple of questions / problems with this fix:

1. The separator between the base URI and item has been changed from '/' to '#'. This breaks some existing code -- we need to discuss why and whether there might be a gentler way to do it.
2. There is a problem with aliases in the owl generator. See: tests/test_issues/input/issue_163c.yaml for details. We need to discuss approaches

[cmungall]

hmm I don't recall intending to change anything from / to #...

[hsolbrig]

@cmungall - see https://github.com/biolink/biolinkml/blob/issue-163-redux/biolinkml/utils/mergeutils.py#L70

[cmungall]

OK, changed '#' to '/' (this is only the case where a prefix is not declared)

Looks like I now have conflicts, most of these seem to be generated test files, maybe I shouldn't have included in the PR

[hsolbrig]

We'll catch the conflicts -- @jiaola is going to create a merge action that automatically rewrites them. Thanks!