validation error for formulas starting with minus

[jvanhoefer]

As @yannikschaelte pointed out to me: If a right hand side/formula starts with a minus, the translator/validator throws ScannerError: sequence entries are not allowed here (Since yaml-blocks start with a -, and the validator complains...). I don't know, wether there is a direct fix (but doubt this a bit, since this is how yaml are parsed??).

Possible ways around would be setting the formula either in quotes ("...") or in brackets... Therefore I propose

• [x] Explaining this in Format Docs & evtl. README (known issues)
• [x] catching the ScannerError and throw a meaningfull error there
• [x] Test, if the Model Editor works in this case?

[jvanhoefer]

Other points in a similar direction

• [x] specify, that any id needs to be a valid SBML identifier (and what that means...)
• [x] any formula is parsed by libsbmls math parser (and what that means), currently that's the last point of the format docu. Make this better visible!

[yannikschaelte]

One could reference the sbml specification, but there may also be constraints from libsbml or mathml, so eventually only the successful export to sbml may give full validity.