fix bug in getFittedValues() that throws an error related to column names

[jr-leary7]

• if you pass in a data frame to the cell.meta.data argument that contains a column named cell it conflicts with the creation of a column named cell internally, causing both to be renamed accidentally by dplyr
• solution
  • check if a column named cell exists in the provided metadata, and if it does keep that column instead of the internal one before returning the results