Make MCRichDigitsToRawBufferAlg functional

Makes MCRichDigitsToRawBufferAlg (almost) functional in order to adapt to changes introduced in LHCb!3421 (merged) and fix the dd4hep builds (tests still fail, but they do anyway in master so...)

I say almost, as Boole still requires algorithms that emulate the RawBanks to fill a single RawEvent object, which is not something allowed in the functional framework as input arguments are const. I therefore have to cast this away in order to fill the RICH banks.

Eventually Boole will need to be updated to change how it creates the RawEvent, to remove the need for this, but that is well beyond the scope of this MR.

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