Make gamgee::missing(float) distinguish between missing values and other kinds of NaN values

Non-missing NaN values include the end-of-vector value and quiet NaN.
Previously missing() would return true for these values.

Fixes #157
Merge pull request #266 from broadinstitute/dr_fix_float_missing_check

Make gamgee::missing(float) distinguish between missing values and other kinds of NaN values