ChEBI IDs should always be strings

[sfluegel05]

ChEBI IDs should be treated as strings. However, pandas sees this differently and automatically assigns them to the integer datatype. This leads to issues down the line, e.g., when filtering into train/val/test:

    self._dynamic_df_train = df_data[df_data["ident"].isin(train_ids)]
    self._dynamic_df_val = df_data[df_data["ident"].isin(validation_ids)]
    self._dynamic_df_test = df_data[df_data["ident"].isin(test_ids)]

train_ids, validation_ids and test_ids are lists of strings, so the assignment fails if df_data contains integers.