Add 'Unknown' to Enumerations

[gordondavies]

https://npyc-toolbox.readthedocs.io/en/latest/enumerations.html

Some enumerations do not have some default values when the expected enum option is unknown.

ie, when importing sample metadata - if the 'Sample Type' is blank or not in the expected list (StudySample,StudyPool,ExternalReference,MethodReference,ProceduralBlank), it may be set incorrectly to StudySample - potentially making any downstream analysis imprecise or inaccurate.

Please add another Enum choice:

Unknown = 'Unknown' to all Enums with String Type values

Unknown = 0 to all Enums with Integer Type values

Please make sure in the toolbox code that whenever a blank or NA or null value is encountered, the correct Unknown is assigned.