Style: Indices used in einsum calls

[alilasemi]

There is some level of standardization in the documentation about which letters to use to represent which indices in einsum calls (for example, i for elements, j for quadrature points), but duplicate indices are needed (currently just n for bases but we need probably 3, so maybe m, n, p?). Once a standard is set, then we need to go through all einsums and make sure they use the right letter.