ReadGroup class along with some support in SamHeader.

ReadGroup is a simple class for storing a read group (@RG) record from a SAM header.  This commit also adds a method in SamHeader that parses the header text and gets a vector of ReadGroups.

I tested this within foghorn and it works fine.
if you make them const instead of constexpr then std::string would be okay. 

also, you should use auto and string literals like so: 
`static const const auto ID_TAG = "ID"s;`