Update alpha_diversity.py

Error messages now use option_parser.error instead of print statements.
Update alpha_diversity.py

If a directory is being passed as output path for a single file, leading to an IOError, the error message specifies so.
Update alpha_diversity.py
Update alpha_diversity.py
Update alpha_diversity.py
Merge pull request #1146 from charudatta-navare/alpha_diversity.py-changes

Alpha diversity.py changes