Replace tuple-style cast with pointer/stride access

See merge request !3

Removes tuple-style cast for member access in AoSoA::get<> in favor of pointer
casting to enable vectorization. Also gives runtime, non-templated accessors
to raw data blocks, data member ranks, data member extent, and data member
strides.

Particle Data Structure

See merge request !1

Adds data structures for:
     * Struct-of-Arrays (SoA)
     * Array-of-Structs-of-Arrays (AoSoA)
     * AoSoA variable slices (MemberSlice)