Fix overflow of 32-bit signed integers when multiplying two 32-bit integers: cast to 64-bit integers first, otherwise matrices with number of elements > 2^31 (e.g. a 48,000x48,000 custom kernel matrix) will not work.
Accidental file included
Clang will not take int64_t as an index into a vector, add [] for int64_t in SGVector
Need a third operator[]

Because the uint32_t used to index vectors elsewhere now cannot choose
between index_t and int64_t (before it only had the choice of
index_t=int32_t)
Other try to fix ambiguities: a single operator[] with int64_t
Merge pull request #1731 from koenvandesande/fix_index_t_overflows

Fix overflows of index_t for large matrices