Sample environment .log files #117

The ISISICP writes sample environment-only .log files for each run in addition to NeXus files.

Some beamlines may still need these.

I think that these files, if needed, should be generated by a separate process from the main kafka_file_writer which writes NeXus files. This can probably be a much smaller/simpler process as it only needs to write SE values, and is much less performance-sensitive than the main file writer. For example this could be implemented as a relatively simple Python script/process which only listens to the _runInfo and _sampleEnv streams.