Fall back gracefully when sum factorisation fails.

[dham]

The sum factorisation implementation only works with up to 6 indices in a contraction. There are
circumstances in which an interpolation kernel can exceed this. Rather than failing in this case, we
should not sum factorise.

[connorjward]

As an alternative could we just have (here)

if len(sum_indices) > 6:
    logger.warning("That's a lot of indices you have there, this might take some time")

[dham]

As an alternative could we just have (here)

if len(sum_indices) > 6:
    logger.warning("That's a lot of indices you have there, this might take some time")

Fair.