Add descriptive error to add_alpha_to_mapping_file

The library function was changed to check for the existance of the
specified rarefaction depth; it will now raise an exception if a
non-existant depth is passed. This exception is now catched from the
script interface and displayed using option_parser.error.

The test suite was updated accordingly for this module.

An unnecessary type cast was also removed from the script interface.

This should fix #827
Add available depths to the exception message
Add default depth as the highest

Mean alpha is now retrieve the highest available rarefaction depth when
None is specified. Script usage and unit tests were updated accordingly.

A new option was added to the script interface to indicate when the user
is trying to pass collated data as the input, since --depth is no longer
required to determine whether or not the execution will be over collated
or non-collated data, there was a redundancy that this option resolves.
Fixed typos

Thanks @jrrideout
Change redundant if else statement

Thanks @gregcaporaso!
Merge pull request #840 from ElDeveloper/issue_827

Add descriptive error to add_alpha_to_mapping_file