Phaseplane toolbox - multiple branches of nullclines not supported & buggy

[robclewley]

Presently, the user must micro-manage the computation of nullclines. The find_nullclines() function will only find one branch of a nullcline at a time, regardless of search/starting point resolution parameter n and size of domain. In fact, the function can get confused if starting points converge to different branches and then it may complain about points being "out of order".