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

There is one glitch with keyword arguments: First I didn't allow keyword
arguments at all. This made it possible to check the number of arguments
and to correctly issue a deprecation warning. On the other hand, ignoring
keyword argument might break existing codes, so I added them back in.
Now the check for the correct number of parameters is not reliable any more,
because it's difficult to judge whether all vector components have been
addressed. 

What do you think? What's the future for keyword arguments anyway?