Initial work
boundary/edge issues resolved
further mods
Added testing and fallback for multiple projections
pep8
Correct RasterSource.validate_projection docstring
Refactor SRS determination.
Allow multi-image responses
Bug fixes and review feedback.
Add/update tests
Merge pull request #1 from rhattersley/wms_diff

Update WMS handling
Update heuristics and test result.
Merge pull request #2 from rhattersley/wms_diff

Update heuristics and test result.
Merge pull request #507 from esc24/wms_diff

Ability to add WMS layers to different GeoAxes projections