[Bug]: NEB implementation should switch to `method="improvedtangent"`

[Andrew-S-Rosen]

I am on an email chain with Graeme Henkelman who says the default implementation of NEB in ASE is not correct. The ASE ase.mep.neb.NEB class defaults to method="aseneb", which is supposedly the problematic one, whereas method="improvedtangent" is the correct one. A future ASE pull request will update the default, but I'm posting it here in case that process takes a long time to get merged.

https://gitlab.com/ase/ase/-/blob/master/ase/mep/neb.py?ref_type=heads#L808-823

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[shyuep]

Let's be explicit and specify our preferred default in matcalc. @rul048 Pls change and push,