disallow multiplication of 3+ vectors w/o parens

[ChristopherChudzicki]

Resolves #108

Error message is potentially confusing in large expressions. Would be great if parse_results included the original string so we could show students what string caused the error. One day, maybe... !

[jolyonb]

Looks fine to me.

[jolyonb]

Ah. I've had second thoughts... We shouldn't give this message for vecv/2/pi...

[ChristopherChudzicki]

We don't, but I think I see what you mean. Currently:

from mitxgraders.helpers.calc import evaluator, MathArray

variables = {
    'i': MathArray([1, 0]),
    'j': MathArray([0, 1]),
}

result = evaluator("i/i*i", variables)[0]
# raises
# Multiplying three or more vectors is ambiguous. Please place parentheses around vector multiplications.
# but should raise
# something above not dividing vectors.

raises an error, but it should just give MathArray([1, 0, 0])

raises the wrong error.

[jolyonb]

Haha, ok ;-)