WIP Indexing autograd and tests, new loss functions

[syurkevi]

Adds new indexing functions to address #43. Theses are needed to implement Losses ans SVM from #34 and #6 those changes are next in line. Operator overloading currently avoided in favor of new select_index () and set_index functions(). Will change to preferred solution as part of review. Also brings in google test framework for testing. examples/autograd.cpp has been converted to (non-comprehensive) tests.