Update Proxies for 2.7

Proxies are now tested in dune-common, which uncovered a few missing operators. Rather than trying to implement each operator, try to provide a conversion to value_type, which should work just fine, following the simd::reference proxies from the Parallelism TS. Also, it avoids problems regarding regarding when the left and the right argument of a binary operation both provide an operator.

Addresses: #11 (closed)