ENH: allow to provide required columns explicitly

Summary

Added

• TableImporter now accepts a existing_columns argument which demands that certain columns exist although they may have missing values

Changed

• The converters and datatype arguments of TableImporter now may have keys for nonexisting columns

Test Environment

unit test

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src/caosadvancedtools/table_importer.py

@@ -221,7 +221,7 @@ class TableImporter():
           value check is not necessary.
 
         obligatory_columns : list, optional
-          List of column names, each listed column must exist and must not have missing values.
+          List of column names that must not have missing values.
 
         unique_keys : list, optional
           List of column names that in combination must be unique: each row has a unique
@@ -272,7 +272,7 @@ class TableImporter():
 
         """
 
-        for col in self.obligatory_columns+self.existing_columns:
+        for col in self.existing_columns:
             if col not in df.columns:
                 errmsg = "Column '{}' missing in ".format(col)
                 errmsg += ("\n{}.\n".format(filename) if filename