Anton/fix negative exponents

New default tolerance 0.001% is provided: 
FormulaResponse from '1e-5' -> 0.001% (equal)
NumericalResponse and ChoiceTextResponse (uses NumericalResponse) from 0 -> 0.001%
@Lyla-Fischer There is small probability, that it can be the case that new 0.001% default tolerance will make some old incorrect NumericalResponses to be correct.

Default tolerance is shown in simple editor as ```= answer +- 0.001%```

This PR provides fix for BLD-434 "negative exponents in numerical response problems".

@nedbat and @polesye and @Lyla-Fischer  please review.