Add StorageIndex template parameter to LU, SVD

Currently, LU and SVD use int as the permutation index type. This is a sensible choice, but disables the interface to external LAPACK libraries when its permutation index type is not int, such as int64_t. There may also be legitimate uses for smaller permutation index types, such as int16_t.

The changes are relatively simple, as can be seen here !1152 (merged).