Allow MetricsReader to return point_ids for properties that are associated with multiple property name keys.

[alisonspencer]

Allow MetricsReader._get_point_ids to return point_ids for device aliases that are associated with multiple property keys.

Issue: For some sites and devices, point_ids are associated with more than one property keys -- eg, both ReturnTemperature and ControlTemperature. When retrieving data for one of these keys, the current code compares the length of the two does the following:

properties to retrieve = ['ReturnTemperature', 'ControlTemperature']
values = [{key: 'ReturnTemperature', 'alias': 'evap_X_returnTemperature'}, {key: 'ControlTemperature', 'alias': 'evap_X_returnTemperature'}]

value_alias_list = ['evap_X_returnTemperature', 'evap_X_returnTemperature']
point_map = {'evap_X_returnTemperature': 'abcdefg-123456'}

The existing code compares len(value_alias_list) == len(point_map.keys())
Since the length of values.alias_list = 2 and the length of point_map.keys() = 1, the function throws an error and no point_id is returned for either property.

Comparing the lengths of the sets instead returns the correct point_id and allows data to be retrieved for both properties.