Remove temporary placeholder classes
Add license configuration for BSD-2
Unify and fix license blurbs

This was done using the commands:

    mvn license:update-file-header
    mvn license:update-project-license
Migrate core OPS interfaces into scijava-ops core

This migration is only the start; these interfaces will soon be further
unified and refined.
Add mailmap for better "git shortlog -nse"
Fix many warnings and javadoc errors
ops-descriptors: purge old imglib2-ops dependency

Instead, we temporarily depend on ops-functions, which is the repackaged
version of imglib2-ops. Eventually, core portions of ops-functions will
be migrated to scijava-ops itself, and then ops-descriptors will
probably not need to depend on ops-functions anymore.