Added filtering of stations by variable

[ricardogsilva]

This PR adds information about which observation variables are available for each station, thus enabling the frontend to decide on station visibility depending on the variable that is being focused.

The proposed implementation adds both:

• a listing of variables to each station's representation
• a couple of optional filtering parameters to the list endpoint in order to only list stations for a given variable

GET http localhost:5001/api/v2/observations/stations?variable_name=TR

{                                
    "features": [                    
        {                              
            "geometry": {...},                      
            "id": "c629b574-2924-40a4-9b49-7f075bf1a246",
            "links": [   
                "http://localhost:5001/api/v2/observations/stations/c629b574-2924-40a4-9b49-7f075bf1a246"
            ],                       
            "properties": {    
                "active_since": "1984-01-01",                                                                                          
                "active_until": null,
                "altitude_m": 1642.0,
                "code": "3",                             
                "monthly_variables": [
                    {                                                                                                                  
                        "id": "e6ed3f91-ccb1-4cd4-ac23-3436f20da98d",
                        "name": "TR"
                    }                        
                ],                                                                                                                     
                "name": "Arabba",                                                                                                      
                "seasonal_variables": [
                    {                 
                        "id": "e6ed3f91-ccb1-4cd4-ac23-3436f20da98d",                                                                  
                        "name": "TR"                                                                                                   
                    }                
                ],       
                "type_": "meteo",
                "yearly_variables": [
                    {                  
                        "id": "2c8b978d-d065-4113-8b9a-7bbfdff2562a",
                        "name": "TDd"                                                                                                  
                    }                                     
                ]
            },
            "type": "Feature"
        },
     ...
  ],
  ...
}

---

• fixes #78