Add accessibleForFree to TrainingMaterial and Course profiles

[acka47]

It would be great to filter TrainingMaterial and Courses that are accessible for everyone without a fee. This could be reached by adding an accessibleForFree statement for the markup. Apparenty, it is already used on other Bioschemas (see code query) like Journal, Tool, Dataset or ScholarlyArticle so the addition to TrainingMaterial and Course would also serve consistency of Bioschemas.

A use case would be the indexing of Open Educational Resources marked up with Bioschemas into the OER Search Index. (I am part of the OERSI team.) isAccessibleForFree could work as a first filter for relevant resources. Afterwards we would check the license information.

P.S.: I started to take a deeper look at Bioschemas after @ljgarcia contacted me at SWIB24 where I was co-presenting a talk on "Community-based development of a metadata profile for educational resources" (slides). It is really impressive what you have built with Bioschemas.org and we definitely can and will learn some things from your approach. (Actually, there already were some points of contact/overlap of our two groups some years ago in the context of the "Add /structuredData property" issue over at schema.org where @stain and me were active.)