Implement a new function get_subtype in DataStore

**Is your feature request related to a problem? Please describe.**
Get_subtype gets a class type and returns all the subtypes of the class. We will add cache to store the previous returned values.

**Depends on**
- List issues that this one depends on.

**Describe the solution you'd like**
For a given class, the function will traverse all valid types in _type_attribute, and add all class that is_subclass() return true to the returned subtype list.
Get subtype should have an inner cache structure to store previous query and results.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
- This is part of the data efficiency project
- This PR should be made to the `master` branch.
- After the data tuple class is finished, we may switch to a new branch for integration.