Variable name in fixedpoint_2D function

[nmboffi]

I'm not sure if this matters much, as I can't actually find where this variable is used anywhere, but the fixedpoint_2D class in phaseplane.py has a variable "_classifcations". I can only assume that this is an unintended typo on "classifications" (particularly by comparing with fixedpoint_nD which has a "_classifications" variable). It might not be an issue, but I thought I would bring it to your attention.