std::scoped_lock was introduced in C++17 and is strictly better than
std::lock_guard as it supports locking any number of mutexes safely.
It's also easier to use than std::lock for locking multiple mutexes at
once.
* Renamed LinearDigitalFilter to LinearFilter
* Filter base class removed since it wasn't useful
* C++: std::shared_ptr<> replaced with double parameter
The old headers were moved into folders because doing so avoids polluting
the system include directories.
Folder names were also normalized to lowercase.
