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).