[bgatlas] cell stats

[groupsky]

• Method: GET
• Route: /bgatlas/cell/:utm_code/stats
• Inputs: utm_code
• Requires: authenticated user
• Returns: count of distinct species from monitoring and/or bgatlas2008_species and top 3 users with most distinct species

  data: [{ user: {firstName: String, lastName: String}, species: Number}],
  count: Number

Test cases:

• a cell with no observations and no species in bgatlas2008_species results in count: 0, data: []
• a cell with no observation and one species in bgatlas2008_species results in count: 1, data: []
• a cell with one observation and no species in bgatlas2008_species results in count: 1, data: [{user: ..., species: 1}]
• a cell with two users each having one observation on the same species and no species in bgatlas2008_species results in count: 1, data: [{user: ..., species: 1}, {user: ..., species: 1}]
• a cell with one observation and the same species in bgatlas2008_species results in count: 1, data: [{user: ..., species: 1}]
• a cell with one observation and the different species in bgatlas2008_species results in count: 2, data: [{user: ..., species: 1}]
• a cell with two users each having one observation on different species and a third species in bgatlas2008_species results in count: 3, data: [{user: ..., species: 1}, {user: ..., species: 1}]
• a cell with four users each having one observation results in data.length = 3
• an observation in a different cell results in count: 0, data: []
• unauthorized request results in 401
• unknown utm_code results in 404