Fix manual contour label positions on sparse contours

Prior to this, contour labels were positioned at the nearest contour
vertex, which in some cases (e.g., straight contours) could be quite
distant from the desired location.

This centers the label on  the closest point on contour itself (using
linear interpolation between vertices).
pep-8 compliance for commit 283f358
pep-8 compliance for rest of contour.py
mention contour plot label positioning in whats_new.rst
add simple test for contour label positioning
remove uneeded files
use """ everywhere instead of '''
use np.inf instead of arbitrary 1e10
Merge pull request #1865 from rephorm/contour_label_pos

Fix manual contour label positions on sparse contours