Refactor vec3 class

[hollasch]

Related issues:

• Lighting Oversaturation Problem #93
• inline keyword unnecessary #153
• << and >> operators overloaded but never used #156

Need to refactor the (now common) vec3 class, some in preparation for issues above. For example, this will address some of the color functionality that we'd like to add (by inheritance or something else).

• Separate declaration from definition — create new src/common/vec3.cpp which contains definitions of many of the vec3 methods. Simple function definitions should be defined in-class (implicitly/possibly inlined).

• Eliminate explicit inline keyword

• Cull unused methods

• Other miscellaneous

[hollasch]

See comments in #153 regarding performance when separating declaration from definition for vec3 class methods.

[hollasch]

Resolved in #226, changes in development branch.