Masks: allow more than one week with 'calendar_week' padding

[sjpfenninger]

Problem description

Currently, we prevent masking with extreme or extreme_diff when selecting length other than 1D and n other than 1 by raising a ValueError:

# Will raise a ValueError
mask = masks.extreme(
    data, 'csp', var='resource', how='max',
    length='1D', n=2, padding='calendar_week'
)

Steps to reproduce the problem

There is an inconsistency in processing more than a single day in _extreme_with_padding which needs to be addressed for the ValueError check to be removed.

[sjpfenninger]

Will be addressed by #356