Accessor for getting the list of continuous values associated with an "ITEM." Usually, these vectors will have length of 1, but the "STATES" item may have a list of all observed values.
For continuous data matrices, each cell of the matrix can store multiple "items" such as the average, range, max... This function retrieves the collection of data for the taxon `taxIndex` and the character `taxIndex` and then returns the values for the item designated by `key`
GetItems() returns the list of all possible items
Values of DBL_MAX indicate missing data. An empty vector indicates that the key was not used in this cell.