Add site info to electricity endpoint

[shahardo]

• [x] Add site to the electricity data record
• [x] Add site category to electricity record
• [x] Add to summary, sum for each site category

[shahardo]

For basic query, the summary looks like:

summary: {
 type: "site_categories",
 values: {
  159: 368926,
  163: 6483785.8002408,
  199: 12114009.390051
 },
 timestamp: { 
  min: "1325368800",
  max: "1435697100"
 }
}

When filtering by meter_category:

summary: {
 type: "meters",
 values: {
  290: "262528",
  291: "636376",
  292: "178168"
 },
 timestamp: { 
  min: "1325368800",
  max: "1435697100"
 }
}

When filtering by site category:

summary: {
 type: "meters", 
 values: {
  290: "262528"
 },
 timestamp: {
  min: "1325368800",
  max: "1422741600"
 }
}

[shahardo]

Done