title: Delete row based on nulls in certain columns (pandas)
body: <p>I know how to drop a row from a DataFrame containing all nulls OR a single null but can you drop a row based on the nulls for a specified set of columns? </p>
  
  <p>For example, say I am working with data containing geographical info (city, latitude, and longitude) in addition to numerous other fields. I want to keep the rows that at a minimum contain a value for city OR for lat and long but drop rows that have null values for all three.</p>
  
  <p>I am having trouble finding functionality for this in pandas documentation. Any guidance would be appreciated.</p>