Adding data frame support to gather_species()

[nstauffer]

This is a quick patch that:

• Adds support for data frames as the value provided for the argument species_file for situations where the species list has already been read into the working environment (e.g., a species list has been read in and manipulated, making the original file out of date)
  • Removes the string requirement from the argument species_file in gather_species() and species_join()
  • Updates documentation to reflect the change in the species_file argument
• Updates documentation that indicated that NULL was a valid value for the argument growth_habit_file to correctly indicate an empty character string instead