Add support for 6 character Maidenhead grid locators

[gibmat]

This pull request adds support to WsprryPi to use Maidenhead grid locators that are 6 characters long, rather than truncating any supplied locator to 4 characters. I based things off of the Fortran code in the wsprx code base, and it works for me on my Pi v1 based on reports I see on wsprnet. I couldn't find another implementation to compare with for equivalent output. (The wsprcode utility on K1JT's site won't produce output with a 6 character locator.)

I also did some simple code cleanup and ran everything through clang-format to make it more readable.

73,
Mathias K0WBG

[gibmat]

Added some fixes identified by running through clang-tidy and clang's static analyzer.

[tinypriest]

Hi gimbat -- I've been using your code on an RPi 2 for a few months now and it works well, all WSPRNet confirmations come through with my grid ID showing all 6 characters. Thanks for a much-wanted addition!