Class representing the group of vectors.
Vectors from the same group have same distribution of chunks among nodes.
Each vector is member of exactly one group. Default group of one vector
is created for each vector. Group of each vector can be retrieved by
calling group() method;
The expected mode of operation is that user wants to add new vectors
matching the source. E.g. parse creates several vectors (one for each
column) which are all colocated and are colocated with the original
To do this, user should first ask for the set of keys for the new vectors
by calling addVecs method on the target group.
Vectors in the group will have the same keys except for the prefix which
specifies index of the vector inside the group. The only information the
group object carries is it's own key and the number of vectors it
contains (deleted vectors still count).
Because vectors (and chunks) share the same key-pattern with the group,
default group with only one vector does not have to be actually created,
it is implicit.