Add new Variable term for `electricCharge`

[aufdenkampe]

Reason: There is not a single Variable Name that has a UnitType of http://vocabulary.odm2.org/unitstype/electricalCharge/.
This variable would cover all potential use cases, so is very generic.

• Term: electricCharge
• Name: Electric Charge
• Definition: Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. The SI derived unit of electric charge is the coulomb (C), although in electrical engineering it is also common to use the ampere-hour (Ah), and in chemistry it is common to use the elementary charge (e) as a unit. The symbol Q is often used to denote charge.
• Category:
• Provenance: https://en.wikipedia.org/wiki/Electric_charge
• Provenance uri: https://en.wikipedia.org/wiki/Electric_charge
• Note: Useful for monitoring battery state.

Requesting here because of issue #75.

Related to request from user @neilh10 at WikiWatershed/monitor-my-watershed#494