First draft of Input GUI

The funciton right at the bottom of the code executes when the button is
pressed, and grabs all the values entered into the text boxes. At
present it just prints these to the matlab command line.

Anything you put inside that function ("Button_GetData_Callback") should
execute automatically when the button is pressed. I've also set up all
the values to be global variables, in a pair of structures, if you want
to access them that way.