Added function to check for validity of symbol.

Summary of Changes
------------------------------

1.  Added a new function in DataAccess class to check for validity of a symbol.  The check can be constrained to a  specific list if required using s_list argument.

2.  Added some basic tests to the \__main__ execution path.

3.  Added *.swp to .gitignore file to stop vim's swp files from polluting the repo.