Clarify extent to which filename regex'ing is still necessary

[charles-turner-1]

#378 changed the way that builders work, relying instead on grid info, rather than regexes.

However, the Builders still contain regex patterns: see eg. AccessOm2Builder, as we fall back to getting time information from filenames in circumstances: see get_timeinfo.
Whether we need these regex patterns - or even separate builders - is something we want to think about.

As a result, it looks like we might still have some errant filename based filtering going on - eg. #459. This is far from ideal & needs resolving.

(Potentially) Related Issue Tracker:

• Extend OM3 regex to include additional patterns #459
• New CICE5 names will break the esm1.5/6 builder #466
• AccessESM1.6 Builder #412
• [DATA REQUEST] Add AUS2200 datasets #319
• ACCESS-CM3 Builder #168