v0.2.1 -- Bugfix: revenue_per_conversion in policy_info xlsx output

Bugfix

• Revenue_per_conversion in policy_info xlsx output was often smaller than what it is actually.
• Info given to policies was also affected. This may have affected policies to perform suboptimally.
• Cause: mean(revenues) is not the same as sum(revenues)/sum(conversions) when revenues contains 0.0 when some conversions are zero.
• Now the correct information is computed, provided to policies, and printed to xlsx.
  • Thanks to pbchen for bug reporting