ENH: add charge_rate attribute to battery nodes

[jbolorinos]

Use Case

Battery charge and discharge rates are often appreciably different and we should be able to reflect both properties in our battery model

Solution

Add a charge_rate attribute to Battery nodes in addition to the discharge_rate attribute

Alternatives

Another alternative is to make the capacity attribute a dictionary of the form:

"capacity": {"energy": 2000, "charge": 850, "discharge": 1000}

Which would make capacity attributes more logically consistent (i.e. currently, the battery capacity attribute refers to just the energy capacity of the battery but there are many "capacities")