Change `version_combined` varibable to type `VersionNumber` in Pulseq IO

[cncastillo]

Instead of doing version_combined = 1_000_000*major + 1_000*minor + revision and do comparisons with versions as numbers (e.g. version_combined < 1004000), we can just use version = VersionNumber(major, minor, revision), then we just need to use version < v"1.4.0". As this is passed to seq.DEF["PulseqVersion"] = version_combined it would be more readable as v"1.4.1" instead of 1004001.