Code header/preamble copyright

[stewartboogert]

@lnevay!!

We currently have.

/* 
Beam Delivery Simulation (BDSIM) Copyright (C) Royal Holloway, 
University of London 2001 - 2024.

This file is part of BDSIM.

BDSIM is free software: you can redistribute it and/or modify 
it under the terms of the GNU General Public License as published 
by the Free Software Foundation version 3 of the License.

BDSIM is distributed in the hope that it will be useful, but 
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with BDSIM.  If not, see <http://www.gnu.org/licenses/>.
*/

I suggest we change it to something like.

/* 
Beam Delivery Simulation (BDSIM) Copyright (C) is retained by the original contribution authors or their organisation as defined in the git logs

or

Beam Delivery Simulation (BDSIM) Copyright (C) is retained by the BDSIM authors (AUTHORS.rst) 

I think we really do need copyright assignment for this code... but this is for another day.