Add R2 calculation to phys metrics standard curves

[AHuffmyer]

Add R2 for nls fits

[hputnam]

Interesting discussion: https://stackoverflow.com/questions/14530770/calculating-r2-for-a-nonlinear-least-squares-fit#:~:text=Basically%2C%20this%20R2%20measures%20how,models%20it%20can%20make%20sense.