Expand fastpath to cover more sparse matrix classes

[bwlewis]

Currently limited to dgCMatrix, see https://github.com/bwlewis/irlba/blob/master/R/irlba.R#L275 and https://github.com/bwlewis/irlba/blob/master/src/irlb.c#L476.

Generalize this

[bwlewis]

Anyone want to help with this?

[vspinu]

What matrix classes do you have in mind? One obvious choice is slam. Another is RcppEigen sparse classes.

What is the expected benefit in terms of memory consumption of this? Is it worth it beyond user convenience?

I am coming to this because I am currently blown out of my 16GB memory on my 3GB matrix and my feeling is that because of the bloated Matrix package, but I might be wrong.

[vspinu]

There is also bigmemory/bigalgebra matrix as per #4.