Complete device update

[RedingJeff]

Is your feature request related to a problem? Please describe.
When using update_device to push made changes in a device to Fiware, the device in the iota client is correctly updated, but the corresponding entity is not. Here only new attributes are added, but removed attributes are not deleted and changed values are not updated. Further changed device_settings as transport are not taken over to Fiware.

Currently the only reliable way to take over all changes is to delete the device and the corresponding entity and to repost them.

Describe the solution you'd like
To make the interaction here much cleaner, I propose the method patch_device (linked to Issue 21), that takes a device model object and makes all the needed updates for the user to take over all made changes to Fiware. It will internaly delete and repost the device only if the main device settings (endpoint,..) were changed, else it will use the methods update_device and patch_entity to update the stats.

This issue will also provide the tests to ensure the correctness of the functions (and a test for update_device, that is still missing). Further a few simple quality of live methods as does_device_exist are introduced.

[github-actions]

Branch 74-Complete-device-update created!