Add basic operations for gmx::BasicVector<>

Add basic operations for gmx::BasicVector<>

1. Overload +,-,+=,-= operations for BasicVector
2. Add class methods for dot,cross products, scale, unitv, length,
   sqlength functions (norm, norm2)
3. Add out of class functions for same operations as class methods
4. Add .toRVec, .toDVec methods
5. Provide forwarding templates in vec.h to get correct norm, norm2
   implementation when both headers are included and function is applied
   to older vector style.

Change-Id: I9d0f4490ae51d12842e0f22e9b24b46754fb4f79
Signed-off-by: Alexey Shvetsov <alexxy@omrb.pnpi.spb.ru>