[assembler] use ns aliases + as_imp for more concise code
[localeval] more as_imp(*this) cleanups
[spaces] sync communicator prep. in cg pdelab space
[product.interface] fixes needless protected subclassing

at least I see no reason for it and it prevents subclasses from having
automatic copying
[discretefunction] per thread cache for local

this was actually the last remaining fix for correct error calc in
multiscale