Remove class objects from DataStore

[mylibrar]

Is your feature request related to a problem? Please describe.
DataStore should only store primitive types as well as built-in list or dict. However, we still find some class objects being stored into DataStore. These structures are usually not JSON serializable. We need to remove them and maybe add some conversion layer if necessary.

Describe the solution you'd like
Class objects to be removed from DataStore:

• FNdArray: We will need to refactor FNdArray for conversion.
• Span

Additional context

• This is part of the data efficiency project
• This PR should be made to the master branch.