Fix indentation issue in TelloDifferential.{h,cpp}

[nicholasadr]

These two files are stuck with the legacy vim tabstop=8 setting.

[nicholasadr]

The problem is related to the vim command set expandtab that was introduced halfway through the code development. The command set expandtab effectively replaces tabs into spaces, however it doesn't seem to replace pre-existing tabs. To remedy this, use :retab to convert all legacy tabs into spaces.

The weird indentation that appears in new commits are in fact the right indentation where tabs have been replaced with spaces.

You can check for existence of tabs as indicated by the ^I characters in the output of cat -A [filename].

[MatthewChignoli]

Closed with #41