Total or tropospheric budget diagnotics.

[MartinCussacMF]

Short description of the diagnostic
I would like to implement a new diagnostic calculating the total budget/mass for a given atmospheric tracer (typically o3 or ch4). It also can do the budget for a sub-part of the atmosphere (typically the troposphere) by masking the input data. My feeling would have been to develop a new diagnostic but maybe it would need to be included in preprocessors?
Anyway, I already have some personal scripts to do this I extracted and joined the relevant parts (sowewhat dirty).
This is my first development for ESMValTool so i'm taking any feedback before diving into the code.

burden.py.txt

Branch and pull request
Coming soon