Implement Strategy

[kabasset]

A Strategy is a collection of Action = std::function<void(const Hdu&)>. It is indexed by some ActionTrigger, e.g. PostFileOpen, PreFileClose, PreHduInit, PostHduInit, PostFirstHduAccess (naming too rude, to be refined) and the HduCategory. Possibly, some ActionTag (e.g. int or string) could be used to add or remove actions.

Something like this must be feasible:

Strategy s;
s.add(Compress(Plio()), ActionTrigger::PreHduInit, HduCategory::IntImage);
s.add(Compress(HCompress()), ActionTrigger::PreHduInit, HduCategory::FloatImage);
s.add(ValidateChecksums(), ActionTrigger::PostFileOpen); // default category is Any
s.add(ValidateChecksums(), ActionTrigger::PostFirstHduAccess);
s.add(UpdateChecksums(), ActionTrigger::PreFileClose, HduCategory::Edited);
s.add([](const Hdu& hdu) {
        hdu.header().updateVersion(hdu.header().readVersion() + 1);
    }, ActionTrigger::PreFileClose, HduCategory::Edited);

MefFile f("file.fits", FileMode::Create, s); // default is empty strategy

At some point, shortcuts could be introduced, e.g. s.preHduInit(Compress(Plio()), HduCategory::IntImage);.

For implementation ease, tags could also be replaced with ids, e.g.:

auto plio = s.add(Compress(Plio()), ...);
...
f.disableAction(plio);
f.initImage(...);
f.enableAction(plio);

Given that such local behaviors should be used with care, such an extra complexity would probably be acceptable.

[kabasset]

Done for compression, with simplified CompressionStrategy, which is the main target of the issue.

[kabasset]

Done with simpler interface, yet to be optimized (avoid reading all headers that often).