This is accomplished by providing a wrapped reference to the
SignalProcessing instance of the Rust program as a global Lua variable.
The wrapper class implements mlua::UserData, wraps the relevant
SignalProcessing methods and provides them as Lua methods.
So far, this is implemented for get_energy_in_band() only.
Now an arbitrary number of samples can be read and appended to the FFT
block. Old samples will be dropped such that only BLOCK_LEN samples
are in the memory buffer. Currently one half of the FFT block is
replaced per update.
Thomas Kolb