Disable Travis running examples for now

Need to work out where R is installing package during tests
See https://travis-ci.org/richfitz/rodeint/builds/24855300
Expose utility functions for checking parameter length
target_r now takes same order of arguments (y, t)

Broken ad-hoc compatibility with deSolve in favour of full
compatibility with the deSolve_style argument that converts
a deSolve functions for use.