Consolidate `to_vector` functionality

There are currently two different functions that more or less convert a `TensorWrapper` to a `std::vector<double>`. `to_vector_from_pimpl` lives in src/tensorwrapper/tensor/detail_/pimpl.cpp, and `to_vector` is in include/tensorwrapper/tensor/conversions.hpp. These two should be consolidated into a single function.