Fixes of serialization issues and null-checks

[odev-it]

While using the C# library in our Xamarin project we discovered several issues regarding serialization:

• There were some missing [KnownType] attributes which caused serialization method crashes
• There were some missing [DataMember] attributes that caused wrong Timestamp readings of downloaded data and wrong Temperature SensorType values.

In addition to that, there were some conditions that caused null references (that were unrecoverable) in Logging.cs file regarding the variable "downloadTask". We added null-checks to prevent crashes.

NOTE: the code is already merged with version 1.0.0

[scaryghost]

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Regarding the fixes:

1. What are the reproduction steps to trigger the serialization errors?
2. What are the reproduction steps that have the downloadTask variable set to null during a log download?
3. downloadTask was changed to throw a fault if d/c during downloading in the previous release. Your merge reverts that, see this line in the diff.

[odev-it]

Regarding the point "What are the reproduction steps to trigger the serialization errors?" the steps to reproduce are very simple:

1. configure a Route that involves the modules IHumidityBme280 or IBarometerBosch.
2. call the method SerializeAsync() on the IMetaWearBoard instance

Regarding the point "What are the reproduction steps that have the downloadTask variable set to null during a log download?". The problem is that sometimes happens that this callback bridge.addRegisterResponseHandler(Tuple.Create((byte)LOGGING, READOUT_PROGRESS), response =>
in Logging.cs is being "randomly" triggered even if there's no dowload in progress causing the downloadTask variable to be null. I've reproduced this issue by executing the source code listed in the attached file: StepsToReproduceNullReference.txt

Regarding the point "downloadTask was changed to throw a fault if d/c during downloading in the previous release. Your merge reverts that, see this line in the diff." I just committed the fix. Sorry for that.

[scaryghost]

The null pointer exception is (eventually) caused by calling ClearEntries. You don't need to use it for the typical logging use case so remove it from your code.

That said, the NPE shouldn't happen regardless. Now knowing the root cause, I will be implementing different fix and thus will not be accepting the logging code changes in this PR. Please modify this PR to only be for the serialization code changes.

Regarding the serialization fixes, the code changes look good to me. I have been adding some unit tests that expose the exceptions. When I have confirmed that this PR corrects the unit tests, I will merge it into the code base.

[scaryghost]

I pushed some unit tests out over the weekend. Please update the PR's origin to the latest commit on master.