Constraints for valid EtherCalc

[amir-zeldes]

Spreadsheet:

• Token column exists - it is left most and labeled 'tok' as a header, has no merged spans
• All columns with any filled cell have a header
• All headers should look like [A-Za-z0-9_]+ (tentative suggestion)
• We should either forbid explicit [<>&"] in cells or make sure they are escaped on all in/output comms with EtherCalc

Metadata:

• No odd characters in metadata names, e.g. chapter@type
• Similar escaping measures for metadata contents as EtherCalc cells
• No tabs anywhere in metadata (bad for ANNIS export, TT format)
• Keys and values should be stripped on entry (e.g. accidentally pasted trailing tabs from Excel, new lines..)

Violations of these constraints should throw hard wired errors to the dashboard/editor validation (no rules need to be defined for these to run, and the interfaces can skip other validations until these are resolved).

[amir-zeldes]

@lgessler - let's use this issue to brainstorm data model constraints for GD