Hardware bringup updates

Updates for bringing up Piksi hardware:
* Add function in AcqResult class to return the mean of the best acq SNR for all acquired satellites.
* Add option in Flash.erase_sector arguments to not check for restricted sectors.
* Refactor code that downloads index and firmware files from download.swiftnav.com.
Removing a full stop from a comment, while the same commit adds full stops to two other comments.  Do we want full stops on comments or not?