Improve test coverage

[jfrost-mo]

Something else

Our test coverage is getting lower, and starting to fail the check. We should try to improve it.

• Remove deprecated postage stamp contour plot operator #710
• Remove unneeded fallback code for old recipe step keys #711
• Test vertical plotting with a filename specified #712
• Test running recipes with no collate steps and running with a specified style file #713
• Test handling of masked arrays in convection operators, and load convection test files with fixtures #714
• Test higher dimentional orography handling in convection operators #715

[jfrost-mo]

Its somewhat better now. Still not done the colourbar code though.