Consolidate else/if conditional logic

[sadielbartholomew]

There are lots of locations in the code I have noticed where the conditional if/else logic can be streamlined or improved, so it flows more intuitively and reduces the need for extra levels of indentation, etc.

This is both in the sense of direct small-scale conversions, for example, there is a whole similar block that can be reduced with elif statements here:

cf-plot/cfplot/cfplot.py

Lines 8967 to 8990 in efe94c7

	# Use accumulated plot limits if making a multiple line plot
	if plotvars.graph_xmin is None:
	plotvars.graph_xmin = minx
	else:
	if minx < plotvars.graph_xmin:
	plotvars.graph_xmin = minx
	if plotvars.graph_xmax is None:
	plotvars.graph_xmax = maxx
	else:
	if maxx > plotvars.graph_xmax:
	plotvars.graph_xmax = maxx
	if plotvars.graph_ymin is None:
	plotvars.graph_ymin = miny
	else:
	if miny < plotvars.graph_ymin:
	plotvars.graph_ymin = miny
	if plotvars.graph_ymax is None:
	plotvars.graph_ymax = maxy
	else:
	if maxy > plotvars.graph_ymax:
	plotvars.graph_ymax = maxy

as well as larger-scale changes to the conditional structure. All of which should not change the behaviour of the code in any way, only its implementation, to make it easier to work with and develop on top of.