There was a very nasty bug in the preparation of the block matrix
'p_mat'. The point is that this matrix is created once for all k-points
with the band dimension being the maximum possible. However, only
a part of the matrix is used at every k-point but the orthogonalization
is done for the whole matrix. The problem was that if the number of
bands for a given k-point was smaller than that for the next k-point
them for the next k-point some part of 'p_mat' still contained data from
the previous step, which messed up the orthonormalization. Now, 'p_mat'
is set to zero at each step of the loop.
Also, property 'nion' was added to ProjectorShell since it is used
very often.
The classes ProjectorShell and ProjectorGroup are now defined in
different source files. This makes 'plotools.py' only contain
routines that control the data flows, including consistency checks
and output.
