[refactor] replace all cpp class getters with accessors for consistency

[apaletta3]

Is your feature request related to a problem? Please describe.
There is a mix of getters and accessors in OSTk cpp code, which is unnecessary and adds duplicate code.

Describe the solution you'd like
We could simply refactor all OSTk cpp getters to become accessors that return const references. This will reduce the amount of boilerplate code and also allow the user to choose what they want returned either a copy (const or non-const) or a const reference. But, for ease of use in Python we map the cpp accessors to python getters.