Fixed a few typos in docs (ahem).
Properly test swizzle() constexpr.

Removed constExpressions() test case and merged it to others, reducing
duplicate code.
Math: test constexpr vector conversion.
GCC 4.6 compatibility: this can't be constexpr, don't know why.
GCC 4.6 compatibility: no delegating constructors.

Don't know why the corresponding conversion operator can't be constexpr.
GCC 4.6 compatibility: can't list-initialize array of classes.

Awesome bug. In GCC 4.6 it throws plenty of ungoogleable `-pedantic`
warnings and in GCC 4.5 it fails directly with "error: bad array
initializer". Fallback to initialization using for-cycle.

Hello, performance? You can go home now.
Don't define Query constructors & destructors in header.

AbstractQuery is virtual class, thus the constructors & destructors are
heavier than it looks.
Trade: don't define AbstractImporter constructors in header.

They are heavier than it seems due to inheritance and virtual
functions.
Trade: don't define MeshData constructor etc. in header.

The class has heavy construction/destruction due to all the vectors.
Minor code cleanup (unused parameters).
Code cleanup: shorter angle initializers.
Various code cleanup.