First attempt at a png comparison
Modify the plot testing helper to use abs path.

This change adds a new directory tests/saved_plots to store the png
files to run the plot comparisons against.

Also modified the plot comparision function so if the file does not exist
it will save the png file to the working directory where the user can then
copy the plot to the testing dir.

I also added a plot() test to test_mapbase.
add peek test to mapbase
Fix peek test to get figure from peek call
Import matplotlib in conftest to force backend before anything else.