title: Remove or replace spaces in column names
body: <p>How can spaces in dataframe column names be replaced with &quot;_&quot;?</p>
  <pre><code>['join_date' 'fiscal_quarter' 'fiscal_year' 'primary_channel'
   'secondary_channel' 'customer_count' 'new_members' 'revisit_next_day'
   'revisit_14_day' 'demand_1yr' 'revisit_next_day_rate'
   'revisit_14_day_rate' 'demand_1yr_per_new_member' u'ch_Ad Network'
   u'ch_Affiliate' u'ch_Branded SEM' u'ch_DSP' u'ch_Daily Email'
   u'ch_Daily Messaging' u'ch_Direct' u'ch_Direct Publisher' u'ch_Email'
   u'ch_Feeds' u'ch_Native' u'ch_Non-Branded SEM' u'ch_Organic Search'
   u'ch_Paid Social' u'ch_Site' u'ch_Special Email' u'ch_Television'
   u'ch_Trigger Email' u'ch_UNMAPPED' u'ch_Unpaid Social' u'quarter_Q2'
   u'quarter_Q3' u'quarter_Q4']
  </code></pre>