Move calculations to outer loops for speed

Moved some things in grav_tess that could be calculated on outer loops
instead of the inner ones.
Moved things out of loops in grav_tess
Merge branch 'master' into speedup
Merge branch 'master' into speedup
Precompute sin and cos of lat GLQ nodes

Gives a bit of a speedup as well.
Update changelog
Merge pull request #22 from leouieda/speedup

Speedup code by moving things out of loops