Version 1.3.0

An accumulation of small improvements rather than a big new feature:

• Validate count() headers on --validate-only
• Better error message if the user attempts to use the non-existent <\> operator (suggest </>)
• Add `min-ngless-version field for modules
• Add early check that block assignments are always to block variables
• Use ZStd compression for temporary files from preprocess()
• Correctly handle subpaths in samples for collect (fixes #141)
• Add to_string() to int and double types (partially fixes #78 & fixes #81)
• Add read_int() and read_double() functions (fixes #78)