Anton/fix negative exponents

@polesye and @nedbat please review.

This PR contains fix for BLD-434 - negative exponents in numerical response problems

This PR fixes errors like that:
```
for correct answer 1.6e-19, student answer 1.6 *10 ^ -19 was incorrect.
for correct answer 1.6e+24, student 1.6*10^+24 was incorrect.
```