Refactoring of well controls

This PR is part of the preparation work for using the new parser code for everything Schedule related. The current PR does *not* use opm-parser, and the PR is directed towards the master branch on opm-core. Qualitatively the PR consist of two different features:

- We have added tests - in particular test_wellsmanager which will be important when comparing well instances coming from old and new parser respectively.
- Refactoring of the `struct WellControls`:
   - Everything related to well controls have been moved from wells.c / wells.h to well_controls.c / well_controls.h.
   - The `struct WellControls` definition has been moved to the well_controls.c file, and access goes through `well_controls_get_xxx()` and `well_controls_set_xxx()` functions. 

The goal of the refactoring has been to have more control when the wells and well_controls are implemented based on new parser.