Rename PMMatrix >> #decompose to prepare implementation of other decompositions

There are several common matrix decompositions (LU, QR, Cholesky, singular-value, ...), making it undesirable to use the unqualified name `decompose` for SVD.

Some possible naming schemes:
 - `decomposeLU`, `decomposeQR`, `decomposeSV`
 - `LUDecomposition`, `QRDecomposition`, `SVDecomposition`
