Cellml download retry

[kwabenantim]

Fixes #39

[Copilot]

Pull request overview

This PR addresses intermittent CI configuration failures (Issue #39) by making the CellML fetch step more robust during CMake configuration, reducing the likelihood of GitHub rate limiting and ensuring configuration fails fast when CellML retrieval cannot succeed.

Changes:

• Switch CellML retrieval from 14 individual file(DOWNLOAD ...) calls to a single GitHub tarball download at a pinned revision.
• Add bounded retry logic with backoff for the archive download, and make download/unpack failures fatal to configuration.
• Update legacy documentation links in CMake and README to the current Chaste documentation site.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
CMakeLists.txt	Reworks CellML fetching to download + unpack a single archive with retries and fatal failure on error.
README.md	Updates the “Getting Started” documentation link to the new Chaste docs site.
apps/CMakeLists.txt	Updates legacy Chaste CMake build guide link.
test/CMakeLists.txt	Updates legacy Chaste CMake build guide link.

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[mirams]

Is something clever making the tarball and making it available automatically? If not it is probably just a bit of hassle that I won't remember how to do (or that I need to do) when adding a new CellML file. Can we just re-try the individual file downloads?

[kwabenantim]

Is something clever making the tarball and making it available automatically? If not it is probably just a bit of hassle that I won't remember how to do (or that I need to do) when adding a new CellML file. Can we just re-try the individual file downloads?

It's GitHub making the tarball using the same url on the repo page (I've updated this now.)

I can make it fetch individual files again if that will be easier to maintain.