add an option in Array.set_labels to ignore missing labels

[gdementen]

When applying the same map (for example to translate several arrays for final reporting), it is very tedious to have to compute the correct mapping for each array (intersect the global mapping keys with the axis labels).

>>> arr = ndtest(3)
>>> arr.set_labels({'a1': 'A1', 'a3': 'A3'})
ValueError: 'a3' is not a valid label for any axis:
 a [3]: 'a0' 'a1' 'a2'

Since this could hide errors and it sort-of-conflicts with #906, we cannot do this by default. Having an option would be nice though.

[gdementen]

FWIW, I needed this for dc2019