- Added a way to handle external layers in the acceptedValues for layers, sending an error feedback when users try to add an external layer which doesn't have a scheme
 - Small correction to the backgroundLayer acceptedValues, where we were not looking at the right place for information, causing the background layer to be void as soon as we tried to reload.

small adjustments

- We now return an object containing  a validation Boolean, errors and warnings to be pushed
- We need to implement the 'addErrors' and 'addWarnings' methods
- Simplification of the layers validation : we no longer give the full set of parameters, since the only thing that we check is the layer id / base url

…e rather than being a method of the AbstractParameterClass, as this couldn't be used in the basic storesync parameters

- we removed some artifacts from when error queues functions were a part of abstractParamConfig class

… when trying to populate the store

 - the timeslider, when reloading, would handle the parameter as a string, and would fail the Number.isInteger test. Also, when activating the timeslider through the button, it would trigger a URL change, and detect some parameters (crosshair and camera, for example) as being present but undefined, causing errors to appear.

small update : ensuring we compare sr  as a number

small update: featureInfo is case insensitive

 - Removed double ternary operator in LayerParamConfig for easier reading
 - Removed dirty ssssssnake case in errorQueues
 - Comments should be in better places, and/or explaining some not so evident parameters validations

Issue : the validation returned a falsy value when the layer parameter was set to an empty string. Also, setting the layer parameter to en empty string would give you the store content, as it was ignoring the parameter in those case

Fix : add a check for an empty string at the very beginning of the layer param validation, and added a check to ensure we can populate the layers with an empty string

removing some snakes