Added in explicit conversion to string type.

[J-81]

Addresses issue when investigation identifier is interpreted by pandas as a numerical type and subsequently rejected when investigation identifier checks the type.

[Zigur]

Thanks for the PR. Just a couple of questions:

Why do you also make the conversion for title and description? Could these be numbers as well in your investigations?

Do you also have the same issue with study.identifier?

[proccaserra]

this PR potential without a test on the input to the str() is unsafe.

TODO: add a condition to test that row['Investigation Identifier'] to check if undefined before applying the str() conversion.

in the absence of reply from @J-81 , one can assume that the same value may be passed to the other fields in that data source, hence requesting a similar treatment)