Allow for non-default SparseMatrix template

[ryanlevy]

Currently the only Eigen::SparseMatrix available to Spectra is the default SparseMatrix<Scalar>=SparseMatrix<Scalar,Eigen::ColMajor,int>, and when passed something else (ex: SparseMatrix<Scalar,ColMajor,long>), a runtime error occurs with a rather cryptic EXEC_BAD_ACCESS.
This commit allows for something like

Eigen::SparseMatrix<double,Eigen::ColMajor,long> m;
SparseGenMatProd<double,Eigen::ColMajor,long> op(m);
GenEigsSolver< double, LARGEST_MAGN, SparseGenMatProd<double,Eigen::ColMajor,long> > eigs(&op, 3, 6);

[yixuan]

Looks great! I'll take a look tomorrow and let me merge it first. Thank you!

[yixuan]

One more thought: If we restrict to use Eigen 3.3, this issue can be solved nicely by the Map or Ref class, see #16 .