First stable release!

Changelog

• The numeric_range library is now used under the hood. It improves lookup efficiency from linear to logarithmic, and also makes error-checking nicer to deal with.
• The calculate() function returns std::nullopt instead of throwing an std::runtime_error if an input value is not contained in any pieces' bounds. This necessitates using c++17's std::optional features.
• Adds a comprehensive testing suite.
• Bumps up json.hpp version.