Hi!

I've found your project quite useful, and I wanted to offer the (largely cosmetic) tweaks I made that make it a bit more ergonomic.

1. I wrapped the OpenMP pragmas and include in #ifdef _OPENMP, so that code can build without warnings if OpenMP is disabled.
2. I replaced MAX with MAX_VAL. Some vendors use MAX as a macro, and this solved a compilation issue with me on OSX. I believe Windows may also.
3. I templated ProxyObject and SparseVector under the container so that the user can provide their own dictionary (as std::unordered_map is rather slow), and simplified the code using them with the keyword auto.

Thanks for your contribution!