fix issue when df_pw is a vector

[guokai8]

when df_pw is vector the perform_anova will return error:

Error in names(df_pw) <- c("measure", "from", "to", "y", "p") : 
  'names' attribute [5] must be the same length as the vector [1]

[guokai8]

Error will return when grouping_column is not a factor:

Error in data.frame(x = c(which(levels(df[, grouping_column]) == as.character(df_pw[i,  : 
  arguments imply differing number of rows: 0, 2