Fix PrepGR when the dataset is format as a tibble object

The PrepGR() function does not work when the dataset given to the ObsDF argument is a tibble object.
The problem is the same using the ShinyGR() function.

The problem is caused by the fact that an extraction of a unique column from a data.frame returned a vector, but a an extraction of a unique column from a tbl returned a data.frame.

Then, a single-column data.frame given as an argument to the data.frame function keeps the original column name and does not take the new name given in the new data.frame.

Original code:

ObsDF <- data.frame(DatesR   = ObsDF[, 1L],
                    Precip   = ObsDF[, 2L],
                    PotEvap  = ObsDF[, 3L],
                    Qobs     = ObsDF[, 4L],
                    TempMean = TempMean)

Solution :

ObsDF <- data.frame(DatesR   = ObsDF[, 1L, drop = TRUE],
                    Precip   = ObsDF[, 2L, drop = TRUE],
                    PotEvap  = ObsDF[, 3L, drop = TRUE],
                    Qobs     = ObsDF[, 4L, drop = TRUE],
                    TempMean = TempMean)