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
digital: fixed psk and qam constellation table generators. Not sure how these got out of sync.
qtgui: adds menu item to clear max and min hold lines on frequency plot.
qtgui: fixes problem of set name not showing up.

Defaults to blank string now; a blank string does not set the title to save vertical space. Any other value will now set the title of the plotter.
qtgui: changed all examples to start wtih empty name fields.

Don't want to have QT GUI Plot show up for every plot now.
Merge branch 'maint'
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.
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
cmake: adds a NoOptWithASM build type for building with assembly output and no optimizations for better debugging.
blocks: Added multiply_matrix_ff

This block maps inputs to outputs by matrix multiplication.
Also includes QA.
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'