Converted paste into a command

[anushka255]

• Converted paste to a command
• Added io submodule to process input files
• Added test cases for added helper functions
• Added new test files (slice*.h5ad1). Each of these files hold the same information as slice*.csv and slice*_coor.csv. This was just added to ensure that files that hold same information but are of different types provide same computational results

[vineetbansal]

can we just add a test to check the execution of the main function in __main__.py (or even just if {sys.executable} paste3 --version will work? In any case, let's not make exceptions like this (let coverage go down if it must).

[anushka255]

Added a test to check version number and got rid of the --omit....

[vineetbansal]

We have some .h5ad files in tests/data/input already. Can we rename these and have the new tests pass? (modifying old tests if we need to). I'm a bit wary of introducing more binary files in the data folder than we already have in there (especially if the purpose is simply to make sure that the new api picks up the files based on their names).