Revert automated Nextclade dataset retrieval in favour of fixed datasets and regimented updates

[jpalmer37]

Based on discussions last week, the group seems to prefer the notion of having fixed Nextclade dataset versions in favour of the nextclade dataset get that was implemented for clade calling.

Here are some ideas for changes:

• remove the nextclade dataset get command and stick with a simple error message if the dataset path is not provided
• change the metadata.json under the Nextclade dataset path to have a "version" field matching those listed by nextclade (i.e. 2024-07-03--08-29-55Z = latest flu_h1n1 version)
• add provenance tracking (grep "version" ${dataset}/metadata.json | cut -d'"' -f4) to parse this version out of the metadata.json