Cleanup SiRawBankDecoder

• remove the need for mutable (by not claiming const when it is not needed)
• drop redundant copy c'tor and assignment operator
• avoid using a raw pointer to address an unbounded array, prefer span.
• adapt checking for end-of-range to the use of span
• make single argument c'tor explicit
• prefer reinterpret_cast over C-style cast
• remove unneccessary friendship
• replace posBegin,posEnd with posRange, and use posRange in (now range-based) for loops
• add posAdcRange(), reimplement posAdcBegin(),posAdcEnd() in terms of posAdcRange