Change `load()` to `import_metadata` in `MeterGroup`, `Building` and
`DataSet`. #264
First draft of new `ElecMeter.load()`. #264

`power_series()` now just raises a DeprecationWarning and returns a
result from `load()`.

Also implemented:

* `available_physical_quantities()`
* `available_ac_types(physical_quantity)`
Implemented `ElecMeter.load()` and `MeterGroup.load()`. #264
Tweaking docstrings for `load`. #264