[FEAT][API] Get explained variance of avatarization/de-identification model

## Description
This feature allows to get the explained variance of records used to fit the model.

## Request

### Method
GET

### URL
`/variance/{job_id}`

### Request Parameters
| Parameter | Description | Required | Example |
|---|---|---|---|
| job_id | avatarization or privacy job id used to fit the model | yes | `0123456789abcdef0123456789abcdef` |

### Request Body
(none)

## Result

### Result parameters
(none)

### Result Body
```go
type ExplainedVariance struct {
Raw []float64 `json:"raw"`
Ratio []float64 `json:"ratio"`
}
```

## Example Curl Request

### Request
```bash
curl -X GET "http://localhost:8080/variance/0123456789abcdef0123456789abcdef"
```

### Curl Response
```go
{
"raw": [
0.9999999999999999,
0.9999999999999999,
0.9999999999999999,
0.9999999999999999,
0.9999999999999999
],
"ratio": [
0.9999999999999999,
0.9999999999999999,
0.9999999999999999,
0.9999999999999999,
0.9999999999999999
]
}
```

## Go server

### Struct
```go
type explainedVarianceResponse struct {
Raw []float64 `json:"raw"`
Ratio []float64 `json:"ratio"`
}
```

### Handler
```go
func (s *server) getExplainedVariance(w http.ResponseWriter, r *http.Request) {
ctx := r.Context()

jobID := mux.Vars(r)["jobID"]
ev, err := s.db.GetExplainedVariance(ctx, jobID)
if err != nil {
http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
return
}

w.Header().Set("Content-Type", "application/json")
if err := json.NewEncoder(w).Encode(explainedVarianceResponse{
Raw: ev.Raw,
Ratio: ev.Ratio,
}); err != nil {
http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
return
}
}
```
## Links

- [Metrics.get_explained_variance](https://github.com/octopize/avatar-python/blob/0.6.2/avatars/api.py#L1002)
- [ExplainedVariance](https://github.com/octopize/avatar-python/blob/0.6.2/avatars/api.py#L1002)

## Automated Issue Details

Dear visitor,

This issue has been automatically generated from the Octopize project [avatar-python](https://github.com/octopize/avatar-python/) to make SIGO compatible. Please vote with a thumbs up or thumbs down to assess the quality of the automatic generation.

Best regards,
The SIGO Team

Start at the getExplainedVariance handler and s.db.GetExplainedVariance entry points, then compare their behavior with Metrics.get_explained_variance in avatar-python's avatars/api.py. The work is done when GET /variance/{job_id} returns the documented raw and ratio JSON values for the specified avatarization or privacy job.

Written by the indexing model from the issue text.