Add checks for matching NaN

[treigerm]

As raised by @juntyr: We have a check that the compressors adhere to the error bound which fails if the compressors produce non-NaN values in NaN regions but it would be good to have a metric which explicit only counts these violations so that we can separate between error bound violations and NaN violations.

[juntyr]

@treigerm We already do check for matching NaNs, so we can close this, right?