Enforce dtype = str for certain columns

[lucasgautheron]

Is your feature request related to a problem? Please describe.

https://gin.g-node.org/EL1000/lucid/issues/4#issuecomment-1908

Describe the solution you'd like

ChildProject should probably enforced dtype = str for certain columns.
Maybe dtype should be a new property of IndexColumn. This is already what has been done in the experimental #260 branch.

[lucasgautheron]

Actually, I think this move may have undesired side-effects. So, instead, I think we should use double quotes around IDs that we want to enforce as strings; This way, reading the metadata with pandas or through the package would yield identical results...