Fix HTML table representation for Events collections with inhomogeneous parts

Currently, the HTML representation of a table is expecting to homogeneous when an explicit header is provided (checked with np.ndim).

In some case, some events collections could have different keys that might not match the first events keys (used for the header) with will lead to a ValueError due to an inhomogeneous shape:

EventsList([
    Event('foo', time='2000-01-01', a='foo', b='bar'),
    Event('baz', time='2000-01-02', b='baz'),
])

No easy workaround is currently available for now.