Add Strategy setters, inc. in MefFile constructor

[kabasset]

First, it should be possible to set the strategy as a whole, instead of appending actions, e.g.:

f.strategy() = makeEuclidStrategy();

Then, the constructor should accept actions and strategy, e.g.:

MefFile f(filename, FileMode::Create, HCompress());

MefFile f(filename, FileMode::Create, makeEuclidStrategy());

This may be how the default CiteEleFits should be exposed in the public API.