Add LICENSE

Right now, EveryBeam does not contain a LICENSE. Which license should we add? Seems that SKA recommends the (permissive) BSD-3 open source license, see https://developer.skatelescope.org/en/latest/development/cplusplus-codeguide.html#license

Not sure if Astron has an open source licensing policy @offringa @tol, shall we follow the SKA recommendation?