Phaseplane toolbox variable naming expectations

[robclewley]

find_fixedpoints() function expects '_'-based (FuncSpec-compatible) hierarchical variable and parameter names (e.g. 'mymodule_element1_x' instead of 'mymodule.element1.x') from ModelSpec model building. However, find_nullclines() works with the user-friendly '.' syntax. Both should transparently support the '.' version.