v0.15.1

This release improves a few crucial things over the last release.

Support for pre-C++17 compilers should be a lot better again - eos compiles again in C++14 mode on VS 2017, and, although not officially tested in CI, likely also compiles again with the likes of g++-5:

• eos now seemlessly uses akrzemi1::optional on all pre-C++17 compilers, not only on Apple platforms.
• Serialisation for optional also works correctly on all platforms (pre-C++17 and C++17 compilers); and updated cereal to the latest develop branch commit.
• optional now also works within the python bindings for pre-C++17 and C++17 compilers.
• Added eos::cpp17::clamp, which uses std::clamp on C++17 compilers, or a fallback otherwise.

Matlab bindings:

• Updated the eos-matlab-fitting bindings! They now work again in the latest version to use the fitting from Matlab.
• Disabled the eos-matlab-render bindings for now, as they would be more work to update. Having the fitting bindings back should suffice for most use cases.

Other small things:

• A few small improvements to the main CMakeLists.txt (86fbfa1).
• Fixed a missing inline, missing include, added a few consts, a couple documentation updates.