Fail addition of Double.NaN to a TDigest instance.

We rely on the fact that values must be comparable, yet Double.NaN compares
unequal to any value (even Double.NaN itself). So we should reject it instead
or corrupting the internal state of our digest instances.
Merge pull request #21 from jpountz/nan_handling

Fail addition of Double.NaN to a TDigest instance.