Update to version 0.7.0

[OnnoEbbens]

Major update with some major changes:

• add functionality to read BRO groundwater quantity data (https://www.broloket.nl), #72
• rename all attributes from Dutch to English, #78:
  • maaiveld -> ground_level
  • bovenkant_filter -> screen_top
  • onderkant_filter -> screen_bot
  • filternr -> tube_nr
  • meetpunt -> tube_top
  • stand_tov_nap -> values
• create read_... (read_bro, read_knmi, ..) functions instead of Class methods, #84
• add 'source' attribute to Observation objects, #85