Custom tolerance criteria in fuzzy compare

It came up in the new buildbot setup for dumux that will be publicly available soon.

We have vtu files with parameters of different order of magnitude, e.g.

• velocity in the order 10^0-10^1
• concentration in the range of 10^-9

These test often fail the fuzzy comparison when doing calculations on a different architecture, often just a different compiler even. For cases like that (might also occur in ini fuzzy comparison) I suggest the following input mechanism

1e-2 | fuzzy_abs_tolerance <vtu keyname>

where is the parameter name in the vtu file. Can you think of a more flexible solution?