Filewriter fails for some string types.

[thompson318]

When the units are ml/s the file writer breaks with

FileNotFoundError: [Errno 2] No such file or directory: 'waveform-export/2025-12-04.xxxxxxxxx.118.mL/s.csv'
........
pika.exceptions.ChannelClosedByBroker: (406, 'PRECONDITION_FAILED - delivery acknowledgement on channel 1 timed out. Timeout value used: 1800000 ms. This timeout value can be configured, see consumers doc guide to learn more')

Due to the backslash.

• Write a test to make sure that the file name maker function handles backslashes.
• Fix create_file_name. Replace/ with p for per?

[thompson318]

Fixed with #21