Add convenience class for comma-separated-values files

Read and write comma-separated-values files with
ascii.read(‘file.csv’,format=‘csv’)
ascii.write(table,”output.csv”,format=‘csv’)
Does not yet automatically recognize .csv extension
Add tests for CSV in io.ascii
Add CSV test file
Add CSV option to documentation tables
Fixed bugs in sphinx docs
Add Table/Unified interface tests for CSV files
List of formats properly alphabetized
Merge pull request #1935 from jfoster17/add-csv-support

Add convenience class for comma-separated-values file