Consolidate options that control reading blocks

[braingram]

AsdfFile.__init__ accepts:

• lazy_load
• copy_arrays

Whereas asdf.open accepts:

• lazy_load
• copy_arrays
• validate_checksums

It is likely that lazy_load and copy_arrays make no sense outside of opening an existing AsdfFile. Combined with no public method to open a file using an existing AsdfFile instance it probably makes sense to move these into asdf.open or perhaps to consolidate these into a set of 'block access options'.