uhd: unexposed usrp_sink::get_device() and usrp_source::get_device() from SWIG

* multi_usrp class is not exposed to SWIG, so function returned a SwigPyObject and caused a memory leak when Python exits
* It is not practical to expose multi_usrp to SWIG, so simply tell SWIG to ignore it
grc: design time values for object-like variables
digital: using new var_value for constellation variables
grc: allow blocks to dynamically hide some of their ports
qtgui: updating how freq message ports work.

Use absolte frequencies relative to sample rate and center freq.

Double-click does not retune, just sends message. Feedback to input freq can be used to retune.
docs: Corrects or expands documentation comments for various blocks where I have had to read the source code to understand the

- char_to_short and short_to_char scale by a factor of 1<<8.
- What delay actually does.
- File descriptor blocks close their FDs when destroyed.

And one code change:

The firdes transition width check error text is made consistent with other similar messages.

Addresses Issue #686.
Merge branch 'maint'
filter: upping the allowed relative error in freq_xlating_fft_filter's QA from 0.001 to 0.0012 to account for minor precision errors with AVX.
Merge branch 'maint'
Removing trailing/extra whitespaces before release.

We should be more careful about letting these into the code in the future. In emacs, we can use (add-hook 'before-save-hook 'delete-trailing-whitespace).
Merge branch 'maint'
Removing trailing/extra whitespaces before release (for master).

We should be more careful about letting these into the code in the future. In emacs, we can use (add-hook 'before-save-hook 'delete-trailing-whitespace).
uhd: Updated docs, in particular for the command interface
uhd: Added command interface to uhd source, modified command syntax
uhd: Refactored common stuff from usrp sink and source
uhd: Added GRC bindings for command interface
uhd: Added more type checking and flexibility to commands and tags
Windows compatibility fixes

* Fixed usage of Windows thread-naming API, changed minimum Windows version
* Fixed MSVC usage of isnan, round
uhd: Added example for re-tuning through messages
uhd: Added checks for lock sensors.

When operating with an external ref, in MIMO mode or with a GPSDO,
the constructor now waits until the lock sensors report succesful
locking.
If this fails, a warning is displayed using the logger interface,
to keep backward compatibility.