Separate schema language and storage away from NWB format

[rly]

• Move schema language to new repo hdmf-dev/schema-language
• Move HDF5 storage docs to new repo hdmf-dev/hdf5-storage #501
• Move schema validator code to hdmf-dev/hdmf
• Restructure /core and /docs/format to be similar to an NWB extension -- @rly
• Remove /docs/general/

[rly]

HDMF schema language is now in https://github.com/hdmf-dev/hdmf-schema-language/
But this is (slightly) different than the NWB schema language which uses keys such as neurodata_type instead of data_type.

Should I fork hdmf-dev/hdmf-schema-language into a new repo NeurodataWithoutBorders/nwb-schema-language, where the values in namespace_map.yaml and in a few other places are changed to be appropriate for NWB? That way, if hdmf-dev/hdmf-schema-language is updated, those upstream changes can still be merged into the nwb-schema-language fork.

[oruebel]

Should I fork hdmf-dev/hdmf-schema-language into a new repo NeurodataWithoutBorders/nwb-schema-language,

I think that makes sense. Hopefully that will not add too much overhead down the line, but since changes to the language are not too common, I think this should not be big issue.