External projects and default copyright

[Lestropie]

Currently, where a command does not specify a custom copyright notice, the software automatically inserts the default MRtrix3 copyright. While this works for the core, where almost all commands utilise the default copyright, it is not suitable for the compilation of external projects that make use of the MRtrix3 API. IMO, where an external command is built, the presence of a custom copyright should be enforced, just as is done so for the presence of AUTHOR and SYNOPSIS.

Relevant to #2939.