Add new feature Distinct

[StefanosChaliasos]

Handle DISTINCT with aggragates

[theosotr]

At the moment, the query generator will not produce aggregate queries and combined queries containing DISTINCT.

[theosotr]

Merge it, but we need to address the following issues in the near future:

• Aggregate queries with distinct
• Columns in SELECT and ORDER BY clauses in Peewee translator.