MultiDatastream Extension tests

[hylkevds]

I've added some tests for the MultiDatastream Extension.

Fixes #39.

[dstenger]

Fixes #39.

[taniakhalafbeigi]

This PR uses a SensorThingsClient library. It means testing against SensorThings with a library that uses SensorThings but not tested against it.
In this case if there is any change to SensorThings that library needs to be updated as well as the ets-sta10 and that adds more delay. I would say if we want to use that library we'd better move the code here so that there is only one place we need to change.

Also the conflicts should be resolved.

[hylkevds]

If the library falls behind, you can always pull the library into the test suite at that point. Doing that when it's not needed is a (very) bad practice.
If you really want to maintain your own (copy of) a client library, you should do it in a separate project, so that its use is not limited to just the test suite. Otherwise you're just wasting your time with duplicate work.

[taniakhalafbeigi]

What you are saying is correct in general for the libraries and that is why we use libraries in coding.
The problem here is different though. This is the test suite for SensorThings. The OGC test suite should be self-contained with anything related to the standard that it is testing.