add dxdv methods to API, #177
add example of integrate_dxdv use, fixes #177 and fixes #23
replace MWPotential w/ MWPotential2014 in the introduction of the docs, #180
replace MWPotential w/ MWPotential2014 in the potentials part of the tutorial, #180
replace MWPotential w/ MWPotential2014 in the diskdf part of the tutorial, #180
replace MWPotential w/ MWPotential2014 in the actionangle part of the tutorial, fixes #180, [ci skip] bc no of the pushed commits changed any code