Consolidate all attributes into something like self.param

[jikrant3]

Currently (v3.4.2), most stellar parameters are defined as attributes of a Single/System class object. Consolidate the fitted parameters together.

Possible way: self.params.T_A = 5000 * u.K instead of self.T_A = 5000.
self.param can also be a dictionary. This automatically ensures that no repeated values are stored.

This way, printing all parameters becomes easier.