Improve readability of dp kernels
Improve readability of dp kernels

Methods for offset slice

Using non-named return variable causes a signficant impact on performance:
Odioca genome goes from	2m30 to 2m56 with this change (3m26 when using
non-indirection). This is go issue 3315. So a named return is used for
the 'at' method giving performance identical to the manually inlined
code.

Some other semantically insignificant changes made to improve
readability.

biogo/biogo,mnsmar/biogo,biogo/biogo,mnsmar/biogo,brentp/biogo,brentp/biogo