independent multi-dimensional vector/tensor class

[Gioacchino Wang]

Consider this as a (very very) long-term valid reminder, an independent multi-dimensional vector/tensor class is always a better choice than using vec3 from HEALPix. There are some special need in hammurabi about vector operations, an ideal vector/tensor class should include following features:

1. multi-dimensional, 1D(scalar), 2D(spinner, useful for Stokes Q,U) and 3D(vector), up to rank 2(tensor)
2. complete c++11 semantics
3. applied to at least Cartesian/cylindrical/spherical coordinates, and with necessary transforming functions

remarks: learn from vector class implemented in deal.II & HEALPix library.

[Gioacchino Wang]

no time for tensor/spinor stuff, check hvec.h for details