Add `Source name` to metadata from GEO

[khoroshevskyi]

It seems Source name often has important information, that we are missing. We should add this information to our database.

[khoroshevskyi]

After investigating this issue, It looks like this field is most of the time duplicated with cell_line and cell_type fields. So for now it is fine if we won't add this field to our database. In the future if we have more time, and will be reprocessing data, we can add it if needed.