ENH: allow to define datatypes instead of converters

Summary

Previously, converters was the only argument of the TableImporter that defined the columns that should be treated. This implied the problem that supplying colname: int would prevent the very useful parsing of NaNs done by pandas by default.

Now, there is the additional argument datatypes where columns can be supplied if no special converter function is needed (i.e. str, int, etc).

Focus

TableImporter

Test Environment

unitttest

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CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added ###
 - `check_reference_field` function to check whether entities with provided ids
   exits (for example when importing data from a table)
+- added the `datatypes` argument to `TableImporter` for columns that do not 
+  need a special conversion function
 
 ### Changed ###