Podpointclient

* Make timestamp=XXX optional, and off by default
* Greatly improve test coverage

* Add charge duration seconds to Charge allowing for more granular tracking of charging time

* Add `ChargeDuration` to `Charge` as `charge.charge_duration`
* Charge duration is the amount of time during a charge 'session' spent delivering power. Available as `raw` int duration and as `formatted` value e.g. '1 hour 32 minutes', '<5 minutes', '2 hours 5 minutes' etc.
* String-ing a ChargeDuration returns the formatted string and Int-ing a ChargeDuration returns the raw value

* Stop supressing `AuthError` and `SessionError`. This allows upstream clients to correctly handle these.

* Add placeholder values for pods:
* `.total_kwh`
* `.current_kwh`
* `.charges`

* Add `.home` attribute to `Pod` objects