Modify/enhance Egg format

[andrewheusser]

Reorganize the way eggs are represented (but provide legacy support for old eggs):

• remove the separate features field and combine it with the pres field
• store the current pres field as a dataframe of dictionaries of item/features
• add a get_pres_items() method that returns a dataframe of just the pres items
• add a get_pres_features() method that returns a dataframe of just the pres features
• add a get_rec_items() method that returns a dataframe of just the rec items
• get_rec_features() method that returns a dataframe of just the rec features
• add recmat input option
• add analyze method to egg
• save eggs in hd5 format instead of as pickles

cc: @jeremymanning