Create proper Hdu container #55
Description
A proper Hdu container would be useful for integration with the standard lib, while MefFile domain-specific names are nice. MefFile::hdus() could provide access to some HduSeq with reduced yet STL-compliant interface. This would also simplify strategy-handling by factorizing calls. We'd have things like MefFile::primary() -> HduSeq::front() and MefFile::append_image(...) -> HduSeq::emplace_back(ImageInitializer(...)). The underlying container would preferrably be some list rather than a vector in order to avoid invalidating references at HDU deletion.