Store tolerances in `ValidationManifold`

[kellertuer]

For the validation it might be nice to store a tolerance, atol maybe or rtol or maybe even keywords in general

• globally (for all tests)
• locally per function (similar to the activation)

The design/struct change for this should be not too large – maybe a bit tricky to get that nicely represented in keywords – but most functions would need to extract this from the ValidationManifold before calling their is_X functions – that might be a bit of (though merely repetitive) work.