Implemented a general-purpose SIMD-friendly array

- The expression templates have been extracted from the vector class to a new class called
v_array, for vectorizing (in the SIMD sense) non-vector (in the geometric sense)
code. The vector class is now based on v_array.
- A rudimentary performance instrumenter has been implemented.
- A draft for the documentation of basic use of the library has been written.
- A few bugs were fixed.