Feature request: Add axis limits to iteration histories about cape

[nasa-ddalle]

I've added several options for "PlotCoeff" subfigures:

• "YLim": list of explicit min and explicit max to use for y-axis
• "YMin": explicit min to use for y-axis
• "YMax": explicit max to use for y-axis
• "YLimMax": outer bounds for ymin and ymax; CAPE will not plot a y-value below YLimMax[0] but may have a min y-axis value greater than that, and CAPE will not plot a y-value above YLimMax[1]. Also supports using None (in Python) or null (in JSON) to use one of the bounds. E.g. "YLimMax": [0.0, null] will guarantee only positive y-values are shown but not set an upper bound.
• same options, replacing Y -> X