convert_of support for downscaling binary point

Uses generic Xilinx convert block if number of output binary points
greater than input as CASPER block does not support this.
Upscaling of binary point and overflow indication

convert_of supports upscaling of binary point now, so
added here
Bitgrowth support added to butterfly_direct

support added at lower level by having separate input_bit_width and
output_bit_width parameters. This needs to be bubbled up and logic
added to calculate these. Scripts for the moment make these the same
to keep functionality as before.