isFirstFragment: Does a SAM flag indicate the first fragment
Description
Compute whether a SAM/BAM flag indicates a
first fragment. Method is not foolproof, as it ignores
a lot of SAM semantics. E.g the SAM spec says:
"If 0x1 is unset, no assumptions can be made about
0x2, 0x8, 0x20, 0x40 and 0x80".
For our purpose this should be enough, but we should
keep an open eye for a more robust implementation in Rsamtools.