Variant: make qual float and handle missing values

The missing values in htslib are represented as special bit patterns and
the user is given some macros to test if the result they just got is a
missing value or not. While I am not sure that this is the best API for
Gamgee, I replicated that for float just so we can have some
functionality to work with. The final implementation for all other types
(hence the other types of int) is up for debate.

closes #99 
QUALS are FLOATS in the standard and in the implementation. I made a mistake (of the copy and paste kind) when I declared it uint32_t first. 

The NaN bit pattern is very dangerous in my opinion. It will fail calculations and make everything more complicated. We could potentially have a templated generic "is_missing" function in gamgee, but it would be cumbersome to implement (with lots of template specializations) and it doesn't relieve the user from the problematics of doing range calculations when a missing value is present. We would also have to take care of that in all functions that do range based calculations. Annoying.

The other option is to have each field return a sensible value, but that takes the ability to distinguish missing values from the user. Also not ideal. 

Suggestions?