Add in-memory database option

[bcyphers]

In general, we would like to better support the use of ATM as a normal library. It should be just as convenient to use pieces of ATM in another python project as it is to use the system from the command line.

To that end, we should add a database option that is purely in-memory -- and does not leave a file system footprint unless requested. For example, we could use pandas DataFrames in the back-end instead of tables in a SQL database while maintaining the same interface for the Database class.

[micahjsmith]

Another option is sqlite in-memory (:memory:)