Resolve "[writer] disk full response"

Closes #1897 (closed)

Disk space checking during scans:

• Print a warning every 3 seconds when the free disk space < 1GB:
  • the default limit of 1 GB can be changed by the class attribute bliss.scanning.writer.file.FileWriter.DISK_SPACE_WARNING_LEVEL
• Nexus writer goes in FAULT state (which stops the scan) when free disk space < 200MB (polling period is also 3 seconds):
  • the default limit of 200 MB can be changed by the tango device property required_disk_space

These measures are taken because disk-full during HDF5 writing corrupts the file (#2588 (closed)).