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
volk: force use of generic kernel when VOLK_GENERIC env variable is defined.

cmake: insert VOLK_GENERIC into QA codes to only test generic versions of kernels. Helps control precision issues.
volk: adds command-line option to enable/disable use of ORC with -DENABLE_ORC=True/False.

If ORC is found on the system, the default is to use it. If we have ORC but don't want to use it, we can turn it off using this.
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).
Windows compatibility fixes

* Fixed usage of Windows thread-naming API, changed minimum Windows version
* Fixed MSVC usage of isnan, round
filter: command messages to change frequency are all relative to the sample rate, not normalized.

This follows the update to the qtgui frequency message blocks in:
eceee28baeed6ee0bdcb66f36992c2a5851ff3bd
Windows compatibility fixes

* fec/ber_tools: added include and #define necessary to use M_PI
* qtgui: changed erfc to boost::math::erfc
* volk: don't try to align with MSVC
Merge remote-tracking branch 'ncorgan/ncorgan/swig_ignore_multi_usrp' into maint
filter: relax gr::filter::freq_xlating_fft_filter QA precision
zeromq: require zmq.hpp be installed
Merge branch 'maint'
Merge remote-tracking branch 'ncorgan/win/master_fixes'
cmake: update revision to 3.7.4 for release