Replace C-style arrays in MatrixFree

[peterrum]

No description provided.

[drwells]

Thanks a lot for doing this. Seeing arrray references instead of pointers in function calls makes the code much easier to understand.

[tjhei]

Is any of this user-facing and as such not backwards compatible? Looks good otherwise.

[peterrum]

Is any of this user-facing and as such not backwards compatible? Looks good otherwise.

Should be all internal. But let's wait for @kronbichler's opinion!

[kronbichler]

None of those should be directly user-facing and even the use of internal (implementation-dependent) data structures is unlikely to directly access or pass around C-style arrays.