First real commit.
Added the ShardedMatrix abstract class and a handfull of instances.
Added first matrix factorization, SequentialQR, and a testing function.
Performs a reduced A=QR factorization where Q is sharded with the same schema as A and R is a pxp upper triangular matrix.
SequentialQR only works if all the segments are full column rank, so in particular s_i >= p for all i.
DiskShardedMatrix kind of sloppy right now, needs some fixing.