FIX: no more explicit padding required for vectors

Additionally, relying on default values for initialization triggers a
deprecation warning. Added three more vector constructors: filled_ which fills
the entire vector with a given value, zeros_ and ones_.

Closes #28

Typo fix.

Return copies from arithmetic operators, rather than the array itself, for more consistent semantics (reported by István Lorentz)

Only ask for write buffers in create_image and create_buffer if USE_HOST_PTR and WRITE_ONLY or READ_WRITE is requested (reported by Antoine Martin)