Create LICENSE

[pckroon]

Today I feel like Apache.
This is pending approval from Tsjerk and Siewert-Jan. In addition, we need to add the following header to all files:

   Copyright 2018 University of Groningen

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http:https://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.

[pckroon]

In addition, we might also need to look at a contributor license agreement (CLA) where people agree in advance that we (actually people in Marrink-Lab) can change the license at a later date --- as long as it remains open source.
https://github.com/cla-assistant/cla-assistant

[pckroon]

Tsjerk and SJ agree. I suggest we merge this for now, but leave it open so we can add the license headers later on.

[Tsjerk]

Agreed and approved