1.16.1 – misc fixes

Mostly minor things to catch user errors and/or explain things better. No functionality change really, except preventing some questionable usage patterns that lead to lots of complexity.

#430 – AVX issues in certain GCC version
#427 – Prevent saving an index that hasn't been built
#425 – Prevent building an index that has already been built
#426 – Handle lseek failures
#419 – ftruncate issues
#410 – swig update
#405 – Use std::sort

The remaining changes were to documentation/test and don't change any functionality