Investigate whether replacing bson with hdf5 files makes sense for IndicatorANN

As discussed, we should try to replace BSON by HDF5 eventually. That might mean that we have to manually serialize/deserialize the ANN data, but it would be more portable and much safer to exchange NNs that way (BSON is, by default, an *unsafe* format, i.e., it can perform arbitrary tasks on a computer)

_Originally posted by @sloede in https://github.com/trixi-framework/Trixi.jl/pull/632#discussion_r667711019_