Fast Fourier transform in Assembler

[pythoncoder]

Not really a driver for external components but I couldn't find a better place for this.

I've updated this rather old library with considerably improved documentation. There are also improvements to the Python code (the assembler code is unchanged). As an example the test/demo programs contained workrounds for firmware issues which have long been fixed.

It uses ARM Thumb assembler so it's specific to STM processors. Data arrays comprise floats and are processed using the on-chip FPU. On a Pyboard it does a 1024 point transform in 12ms. It supports polar and dB conversion and the use of window functions. It uses the Cooley-Tukey DFT algorithm so sample arrays must comprise 2**N elements where N is an integer. To save RAM transforms are performed in-place (i.e. the input arrays are used for output values).

It supports data acquisition using a Pyboard ADC with simple demo programs for this.

Interface changes are minimal and designed not to break existing applications.