Terra-layer

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* Add initial opacity of layer
* Add insert in tree to ease processing
* Review README

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Fixes

* Field order was not saved
* Don't save layer group on write

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Fixes

* Replace SimpleRouteur by DefaultRouter that handle `format` param

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Fixes

* Remove id to source field translation for fields

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Fixes

* Fix field ordering bug
* Fix migration bug on Group
* Add permission on layers
* Remove code slug is None

Quality

* Increase coverage
* Add a lot of tests
* Add flake8 pipeline travis
* Fix coveragerc

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Fixes

* Fix load layer command

Improves

* Add uuid field for layer to ease import/export