In CSV export, `voters-in-conv` shows counts of total participants instead of grouped participants

[patcon]

Re-ticketed from: #102 (comment)

(cc @derjogi bc you have context on this, but no pressure to be the one who does this! You've contributed more than enough by submitting this original feature :) )

I just realized in this demo of polis export that both voters and voters-in-conv` counts in the summary.csv show as equal:

https://gist.github.com/patcon/c768fb0d009fa5533beaccfe54e5c6b9

In the polis conversation report itself, it should be 65 and 56, respectively: https://pol.is/report/r2dfw8eambusb8buvecjt

The fix should be simple: Just count the number of participant IDs in loader.math_data["in-conv"], and use that for the voters-in-conv value :)

math_data comes from: https://pol.is/api/v3/math/pca2?conversation_id=6jrufhr6dp