Code cleanup from #12551

[AmeliaEScott]

Describe problem solved by this pull request
#12551 was merged before I could address the review of @bkueng

Describe your solution
I addressed the review of @bkueng

Describe possible alternatives
I could ignore the review of @bkueng . But he seems to know a lot more than me about writing good code, so ignoring him would be unwise.

[AmeliaEScott]

@bkueng regarding this review: #12551 (comment)
The reason I kept it as a default parameter is because in most of the places this library was used before my changes, the default constructor (with no arguments) was being used. I would like to keep this as it is, so there are fewer changes to make elsewhere.

An alternative solution would be to have two constructors:

Battery() : Battery(1, nullptr) {}
Battery(int i, ModuleParams *parent) {...}

But I guess that's functionally identical to the version with optional parameters.

[bkueng]

Thanks Timmy :)
The reason I asked for the change is for users of the class to be forced to set parent. If they are not, it can easily be forgotten, and then dynamic parameter updates won't work.
Can you update AnalogBattery as well?