borrow from EAD3 schematron

[VladimirAlexiev]

@lindareijnhoudt: @boychev put your schematron stuff from dropbox here, and will be extending it here.

• We can probably borrow some stuff from https://github.com/SAA-SDT/EAD3/blob/master/schematron/ead3.sch
• in particular, https://github.com/SAA-SDT/EAD3/blob/master/schematron/ead3.sch#L33 does language code checking by loading appropriate lookup XML files.

[boychev]

Why don't we simply use that EAD schema together with ours, instead of copying from it?

[VladimirAlexiev]

Because EAD3 has a different structure, so the xpaths need to be revised.