Implement BFS exporter for spot data
This ticket is about implementing the plug-in for exporting bioassay set data in BFS format. A description of the file format is found in #1440. The implementation of the plug-in should build on/use the generic writers that are implemented as part of #1444.
This ticket in itself can also be split in two parts. First, a working export implementation (not a plug-in) is created, then a plug-in is created that uses the exporter. This is similar to how the BASEfile export and plug-in was setup as part of #742.