Allow an in-archive schema file

[xuhdev]

Permit an archive to store a schema file. We can put such a file inside a DAX or other dataset so that if a user predownloaded it, pydax can simply load this dataset by reading the schema file in it. We need to develop a policy on potential clash.

This is relatively unurgent at this point.

Suggested by Luciano

Not necessarily in the first release, but if time permits, we should do this.