StrawDigisFromArtDaqFragments needs cleanup

[brownd1978]

this code has developed organically while debugging artdaq issues and could use a cleanup. The main issues are:

• the code uses a mix of c++ and c-style casts, with types and const not being respected. A consistent use of c++ casts would be better. Even better would be if the fragment-related objects provided casting functions themselves, though that is outside the scope of this repo.
• the module uses data members to cache data descriptions (nSamples_ , np_per_hit_, etc) which are set on reading the first valid packet, but aren't reset between runs, where they can change
• the decoding is spaghetti code. A more encapsulated set of functions would be easier to maintain and debug.
• the bounds checks aren't complete, and are interspersed with the decoding. Factorizing out the bounds checks (per fragment, per DTC, etc) would make the code easier to understand.