Make index type of Triplet default to SparseMatrix::Index as suggested by Kolja Brix. Fixes Bug 665.

--HG--
branch : 3.2
extra : source : 6d0a7ec5f2cd4d628e56293ca2beb9aa6fb461c0
Use != instead of < to check for emptiness of iterator range (fixes Bug 664)

--HG--
branch : 3.2
extra : source : 2261621dfdcdd7f48526f77df2b5d812fc84e9e7