allow user-specified unrelaxed shear and bulk moduli

[jbrussell]

One can specify unrelaxed bulk and shear moduli (such as from perplex) directly. The code checks for shear and bulk modulus. If only shear modulus exists, then bulk is calculated with a warning about the assumed value of nu. If neither exist, the reference modulus is used as before.

[chrishavlin]

I'd like to add a new test for this before merging. @jbrussell if you wanna try, there are instructions at https://github.com/vbr-calc/vbr/blob/main/vbr/testing/README.md but it's a little convoluted so i'm happy to write it and push up to your branch here. No need to fully tie into perplex in the test: just setting the new G_TP and K_TP variables to arbitrary values to make sure they get used would be sufficient.

[jbrussell]

I think it would be best for you to do the test if it's easy enough? I'm not familiar with the process and there's a good chance I would mess something up...

[chrishavlin]

haha, ok, no problem, I'll add it!

[chrishavlin]

before I do, it'd be good to put in the changes I suggested above -- if you hit the "commit suggestion" button it should make the changes for you (but then you'll want to run git pull locally to update your local version of the branch with those changes)

[chrishavlin]

@jbrussell what units are your perplex G, K in ? Gu_0 here is expected to be in GPa, so there's a conversion in el-calc_Gu_0.m. I'm OK with G_TP and K_TP being in GPa or Pa, just need to make sure that what comes out of this function is in Pa

[chrishavlin]

Ok, i have a test ready to go, I'll push it up after you incorporate my suggestions

[chrishavlin]

thanks @jbrussell !