Allow finding LHCb::Digit MC associations only by digit index

By default, the CaloDigit2MCLinks2Table algorithm will ask the linker tables created in Boole for links based on the index and location of the input digits. This means that if one clones digits and then runs CaloDigit2MCLinks2Table on the clones, no relations will be fonud because the cloned digits have different TES locations to those defined in the linker tables under Link/.

This MR adds a new property to CaloDigit2MCLinks2Table that allows for association of digits their index only. This is useful because the microDST cloner framework does not modify indices when things are cloned.

The use case for this is Tesla, where we create MC relations tables. Because the cloning of digits has already happened in Moore, we need relations table makers to take the cloned containers as input.

/cc @ibelyaev (as author of CaloDigit2MCLinks2Table) @rmatev