Merge ska-sa into HEAD (using imerge)

SKA-sa changes (mostly FFT/FIR speed optimizations)
Merge branch 'ska-sa-merge'
Changed bram optimization in delay from "Speed and Area" (which doesn't appear to be valid) to "Area"
Merge branch 'ska-sa-branch'
Added option to implement convert adder in a DSP slice. Inspired by DM, casper_library_misc.mdl was editted directly as a text file.
Added DSP adder support to convert_of blocks, and bus_convert
Added new xps class for a 'ucf' yellow block (block to be added shortly). This class allows custom constraints to be added to a compile.
Added UCF block, which allows custom constraints to be injected into a compile, much as the PCORE block allows custom pcores.

Under the hood this block uses the toolflow infrastructure to inject constraints via the gen_ucf method
Added options for casper convert to be implemented in Behavioural, Fabric or DSP cores.

Propagated these options to the bus_convert block (which now also has an option to override casper convert block with a xilinx cast).
Propagated these options throughout the FFT library, also allowing different latencies for convert and add operations to be used in different parts of the FFT