Make MefFile accessors names more standard about elefits

[kabasset]

After careful consideration, as() looks a bit awkward. Access is more expressive to me, and:

const auto hdu = f.access<ImageRaster>(1);
// Access the image data unit of index 1

reads better than:

const auto hdu = f.as<ImageRaster>(1);
// As an image data unit, access the HDU of index 1

Standard tuples use get() which would be a bit inaccurate here, as what we "get" is not a data unit but a handle to it, and the MefFile is the owner; access doesn't convey the message that the user gets ownership.

find() is fine, though.

[kabasset]

A bit of inspiration:

• from Python's ElementTree: operator[](), get(), find(), findall(), append(), insert(), remove(), index(), iter(), iterFind();
• from nlohmann's json: operator[](), get<>(), at(), iter()