add_bathymetry method for oo section plots
replace SectionAxes subtypes w/ function dispatch

it makes sense to want to layer multiple fields interpolated in different ways
on a single axes, which previously wasn't possible

this simplifies the type hierarchy, at the expense of making
`_interpolate_section` into something of a mega method

the BaseSectionAxes/SectionAxes distinction is no longer necessary, but I've
kept it for now
add experimental object-oriented ts plots
bugfixes