Some bug fixes and improvements

1. When using higher order modulation schemes the transmitter will stuff multiple packets from the TUN interface into a single OFDM frame so that the symbol length of the frame is consistent. The receive side was writing it to the TUN interface as a single packet which resulted in only the first packet actually getting through, severely reducing the perceived throughput. This issue has been addressed and throughput now scales as you would expect with modulation order.
2. Made logging more efficient by opening and closing the file once rather than each time it is written.
3. Changed rx worker to use a single call to the ofdmflexframe rather than an unnecessary loop and moved the USRP recv call out of the thread locking which helps with cleaning up the thread at the end of execution.
4. Added changes made by Manolis Surligas which address the potential for EVM to be NaN in the FEC and Mod adaptation CE's.
5. Fixed some typos.