Updated ComponentPage 

Updated component page to simplify testing variables. Flags can be passed to `get_editor_page` method to use the new or old style component page. New style will return a list of `Variable` objects for `get_inputs` and `get_outputs`. A single variable can also be obtained with `get_input` and `get_output`. 

`Variable` objects have attributes per named columns in a slickgrid table. By default, these include `name`, `units`, `description` and `value`. Calling any property returns a `GridCell` object. 

A `GridHeader` class was also added to `pageobjects/grid.py` and holds the Selenium webelement for a specific column heading. This class can be further augmented to support testing context clicks and other behavior with slickgrid column headers.