Naming convention: abstol -> atol, reltol -> rtol

[gabrielgellner]

I notice that scipy uses the shorter version. As do some Julia libraries. But I think Optim.jl uses the longer names. Need to decide which to support.

[gabrielgellner]

Cubature.jl uses reltol, abstol
As does the builtin Base.quadgk. So the longer names it is!