Some improvement ideas

[laurent-thiebaud-gisaia]

Few things we could do or try to lake it more perfect:

• for better readability, some checks can be done with Apache libs e.g.
  if (collectionReference.params.excludeFields != null && !collectionReference.params.excludeFields.isEmpty())
  can be replaced with
  if (StringUtils.isNotEmpty(collectionReference.params.excludeFields))
  or by filtering empty spaces with
  if (StringUtils.isNotBlank(collectionReference.params.excludeFields))
  There is also a CollectionUtil to handle collections...

• tests are cool, but DatasetTool and CollectionTool with their static methods look outdated. Is it interesting to rewrite them? Is there any value to use https://www.elastic.co/guide/en/elasticsearch/reference/current/integration-tests.html ?

• The CheckParams doesn't only check the params, it also set some default values. It is ambiguous, maybe the roles can be splitted into 2 classes or the class should be renamed?

• Some classes a few methods but a lot of code with a huge cyclomatic complexity, this can probably be splitted into multiples classes or/and shorter methods