Change default format from ply to obj

[JiajunZhang-neu]

Related to #17
Reading .ply files depends on the Rvcg package, which is not installed by default with bancr; if missing, functions may return an empty neuronlist without throwing an error. In contrast, reading .obj files does not require Rvcg, making it a more robust default workflow.

[alexanderbates]

Thanks @JiajunZhang-neu — landed your fix directly in fafb442 along with an explicit requireNamespace("Rvcg") guard so an explicit format = "ply" call raises a clear error instead of returning an empty neuronlist. NEWS credits the diagnosis. Closing in favour of the merged commit.

[jefferis]

Obj files are quite a bit heavier than ply - I would have standardised on ply myself. You can just suggest Rvcg if you want. I don't know how you're reading obj but the reader in rgl is a bit flaky and the readobj package is also an external dependency.