Pull Instruction Group Logic Closer Together and Simplify

[dANW34V3R]

When adding a new Instruction Group you need to change the following files: InstructionGroups.hh, ModelConfig.cc and Instruction.cc::getGroup(), adding repeated information. Numbering is required to be contiguous which combined with the base + offset methodology of getGroup enforces nonsensical groups e.g. FLOAT_SIMPLE_SHIFT or more complex checks to ensure these aren't returned by getGroup. Ideally, a new group would only need to be added in one file and nonsensical groups wouldn't be required.