Rename application variables

[landinjm]

@fractalsbyx Any in particular?

[fractalsbyx]

I dont know much about most applications. In grain growth, I think 'KnV' and MnV' are awful model constant names for the multiwell coefficient m and gradient energy coefficient kappa, respectively.

[fractalsbyx]

Actually, I think every variable in gg should be renamed.

[landinjm]

If you can start a list that would be great!