Add asserts to scored_method, infinity HACK to psi voting with sum = 0.

Also note a bug in QPQ: it seems like it's reading ratings, and it
shouldn't be doing that...

src/multiwinner/methods/exhaustive/psi.h

@@ -53,7 +53,7 @@ double digamma(double x) {
 	//  Check the input.
 
 	if (x <= 0.0) {
-		throw std::invalid_argument("digamma: x < 0 is not supported");
+		throw std::invalid_argument("digamma: x <= 0 is not supported");
 	}
 
 	//  Initialize.
@@ -99,6 +99,12 @@ double psi_voting_eval::evaluate(combo::it & start, combo::it & end,
 		norm_rating_sum += this_ballot.get_norm_score(*pos);
 	}
 
+	// digamma(0) = infinity. HACK to deal with this without having
+	// to bring in infinities.
+	if (delta + norm_rating_sum == 0) {
+		return 1e9 * this_ballot.weight;
+	}
+
 	return digamma(delta + norm_rating_sum) * this_ballot.weight;
 }
 

src/multiwinner/methods/exhaustive/scored_method.h

@@ -15,6 +15,10 @@ class scored_ballot {
 		std::vector<double> scores; // #voters by #cands
 
 		double get_norm_score(size_t candidate) const {
+			assert(min <= max);
+			assert(scores[candidate] >= min);
+			assert(scores[candidate] <= max);
+
 			return renorm(min, max,
 					scores[candidate], 0.0, 1.0);
 		}

src/multiwinner/methods/qpq.h

@@ -1,10 +1,13 @@
 #pragma once
 
 #include "methods.h"
-//#include "../tiebreaker.cc"
 #include <vector>
 #include <list>
 
+// KNOWN BUG: Does not handle negative scores well. I have no idea why,
+// since this is a ranked method. Use multiwinner_spatial with Euclidean
+// distance and no noise to reproduce.
+
 class QPQ : public multiwinner_method {
 	private:
 		ordering::const_iterator ballot_contribution(