0.18.0 - Search 2.0

Fixes

• Fixed bug preventing to use Compound operator with Search and SearchMeta classes.

New Features

• Pipelinized Search and SearchMeta classes. That is complex expressions can be built step by step by chaining operators.
• Updated search method in ¨Pipeline class to ease the use of the search stages.
• Clarified and simplified faceted search

Refactoring

• Use operators rather than statement in Compound class
• Factorized Search and SearchMeta classes by creating a SearchBase class
• Use CountOptions rather than raw dicts
• Created AnyStage union type

Docs

• Spelling and grammar fixes