pmt: Added float conversion

Added to_float() and from_float(). These are basically aliases
for *_double() with a type cast for when strict typing is necessary
(e.g. SWIG won't accept an f32 value when using from_double()).
Add vector support to abs block. Remove unused num_inputs from xml.
blocks: Added callback to vector source GRC bindings
Fix CMake abuse for assembly (.s) files
Merge remote-tracking branch 'willcode/master'
blocks: make tagged stream to pdu output optional
volk: add feature for json results export
Merge remote-tracking branch 'geiger/master'
Merge remote-tracking branch 'nwest/volk-json'
Merge branch 'maint'
Merge remote-tracking branch 'osh/tags_wip'
runtime: clean up tag pruning using iter range