Create a method that allows us to add to WELLLIST

**Is your feature request related to a problem? Please describe.**
Lack the ability to add to a WELLLIST or create a new well list for a given datetime.

**Describe the solution you'd like**
Use some of the add_operations in the network file to be able to add a Welllist. Will need to make Welllist inherit from DataObjectsMixin.

The required solution will need to take a welllist name, list of wells to be added and result in the following being added to the network file.

`model.network.welllists.add(welllist_name='welllist_name', wells=['well1', 'well2'], date='01/01/2020')`

> TIME 01/01/2020
> WELLLIST welllist_name
> ADD
> well1
> well2
> ENDWELLLIST
>

**Additional context**
This is required to be able to add an RFT at a specified datetime.