[x] The Data base class needs to be extended with a comment read only property which gives back either the comment column from a dataframe or the comment field entries from the MSP file
[x] Implement the comment property in the Data implementations
[x] Extract the RI information from this Series/array
[x] Use the setter on the retention_index property to write the data back
Database class needs to be extended with acommentread only property which gives back either thecommentcolumn from a dataframe or thecommentfield entries from the MSP filecommentproperty in the Data implementationsretention_indexproperty to write the data back