TODO: validation

[borismarin]

we won't do much semantic analysis for the time being (tacit assumption: we are being fed 'valid' modfiles and just want to do codegen), but if we wanted to build a proper compiler, it would be important checking for:

• all vars in exprs defined
• scoping
• units (modlunit)
• all vars in STATE have the corresponding derivatives in DERIVATIVE
• SOLVE solves DERIVATIVE with known METHOD