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.

[dham]

Result of Firedrake meeting discussion:

• Make the maximum sum factorisation index be a variable.
• If that variable is exceeded, don't sum factorise but do issue a warning. The warning should specify how to set the sum factorisation index.

[dham]

Actually the proposed solution is not great. We'd have to pass the parameter through a whole load of code.

Additionally, this changes the generated code, so in principle we should change the signature of that code.

I think this is a mess for what is basically a hack in the first place.