rename tissue_type into  sample_type

The property "tissue_type" allows the following values: "tissue", "organoid", and "cell culture". With the introduction of BCP data, we would need to add "cell line", and I think we would be misleading the users with the property name "tissue_type". I suggest renaming the property to a more generic "sample_type," mitigating confusion and allowing indication of a broader range of sample types. 