volk: cleaned up 32fc_x2_dot_prod_32fc code. Improved results; fixed on OS X.

- Added unaligned versions of kernels.
- Fixed issues with tail handling
- Use _mm_setzero_ps instead of subtracting to zero out values
volk: fixed issue with size_t comparison of std::string function (compare against std::npos, not -1).
filter: fixed issue with resampler using buffer size information (always a mistake).

The intent was to ask the scheduler for the maximum amount of data. Now, it plays it safe by asking for just enough the next time. Performance tests show it works well.
qtgui: enables use of QWT 6.1.