Array type inference issue for mixed magnet classes

[gupichon]

4. magnets[:] and magnets["QD"] works but return ElementArray instead of MagnetArray

type(sr.design.magnets[":"])
>>> pyaml.arrays.element_array.ElementArray

5. Similar bug with using subset of magnet arrays

magnets.get()        -> MagnetArray     ✅
magnets.get()[:]     -> ElementArray    ❌ should be MagnetArray
magnets.get()['S*']  -> ElementArray    ❌ should be MagnetArray

Originally posted by @GamelinAl in #376 (comment)