F gaps in int columns

Summary

For #126 (closed). Adds support for empty values in integer columns and Pandas-specific datatypes. This is needed for https://gitlab.indiscale.com/caosdb/customers/geomar/management/-/issues/269.

Focus

The most important part is to use the dtype argument in Pandas' read_csv function. We also replace the np.issubdtype and isinstance functions in the datatype checks by helper functions that support the relevant Pandas datatypes.

Test Environment

For the actual bug, the new unit test should be sufficient. Also use this branch in the Geomar server-profile and the bis-custom submodule in branch f-nan-in-int and test a sample upload with empty fields in, e.g., the integer-valued AphiaID.

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