Da abstract classes (#579)

* created AllDA class to perfrom all DA actions

* added additional packages to be imported

* fixed indentation errors

* cleaning up the code a bit

* added update function and some small bug fixes

* fix update function, added documentation for functions and AllDA class

* fixed typos and moved build_lastobs_df function into DataAssimilation.py. Also simplified this function

* moved read_reservoir_parameter_file function into DataAssimilation module

* changed usgs/usace lake gage variables and waterbody_type_df to be called from within network object (now agnostic to type of network).

Co-authored-by: Sean Horvath <shorvath@cheyenne4.cheyenne.ucar.edu>