refactor indexedviewmethods, enable non-const ref access with symbolic indices

Reference issue

Fixes #2630 (closed)

What does this implement/fix?

Indexed view expressions that accept a symbolic index (i.e. placeholders::last) currently return by const reference or value only (CoeffReturnType). This patch allows appropriate l-value expressions to return scalars by non-const reference.

Also refactors IndexedViewMethods.h so that it is not included twice, and instead defines a const and non-const version of each function.

Additional information