Search function refactoring and adding tests

[cedric-famibelle-pronzola]

• Search function refactoring : (524800ab8aef2cb2f1971d872752d0890b018c7d)
  • Create functions: formatParams and createFeatureCollection.
• Add dependencies needed for testing and coverage visualization: ava, supertest, c8 (8aa178eed16085f8b77c4d71f2857aad33fdc27b)
• Add test scripts and Ava configuration in package.json (6cb7e78efb8e828022782552af5dfa06e5699e7c)
• Tests w and errorHandler (338b4123ca2cb2ba725f4a0894278588af72f937)
• Create mockCluster (02ea56a383c4e5f0ec0ead89a1859c6a4f93fd4e)
• Tests functions used in lib/search.js: ensureSingleValue, formatParams, createFeatureCollection (790f44ac952006f197b1b6bc8c831aa56931ca05)
• Move routes from server.js to lib/routes.js (369936be9cd4cac76b09939b4fafaabe3ee770d4)
• Tests /search and /reverse routes (f23eab3eb1c8a1a724ecaba10cfd646589d50a74)