Matrix.pm: use lower precision in summary stats

"%6g" precision should be sufficient, while also
speeding up the summary stats calculation.

Update some test expectations accordingly.

lib/Biodiverse/Matrix.pm

@@ -400,7 +400,8 @@ sub get_summary_stats {
     my %data;
     \my %top_level = $self->{BYELEMENT};
     foreach my $href (values %top_level) {
-        $data{$_}++ for values %$href;
+        #  round to save time and space - approximate vals should be OK for this
+        $data{0 + sprintf "%-6g", $_}++ for values %$href;
     }
     my %r;
     my $stats

t/26-Cluster2.t

@@ -147,14 +147,14 @@ sub test_phylo_rw_turnover_mx {
     my $mx = $mx_arr->[0];
 
     my %expected = (
-        PCT975 => 0.8059701493,
-        PCT95  => 0.8059701493,
+        PCT975 => 0.80597,
+        PCT95  => 0.80597,
         MIN    => 0,
         PCT025 => 0,
         PCT05  => 0,
-        MAX    => 0.8059701493,
-        MEAN   => 0.3751810522,
-        SD     => 0.3047706206,
+        MAX    => 0.80597,
+        MEAN   => 0.3751808,
+        SD     => 0.3047705823,
     );
 
     my $stats = $mx->get_summary_stats;
@@ -197,14 +197,14 @@ sub test_rw_turnover_mx {
     my $mx = $mx_arr->[0];
 
     my %expected = (
-        PCT975 => 0.8333333333,
-        PCT95  => 0.8333333333,
+        PCT975 => 0.833333,
+        PCT95  => 0.833333,
         MIN    => 0,
         PCT025 => 0,
         PCT05  => 0,
-        MAX    => 0.8333333333,
-        MEAN   => 0.3923076923,
-        SD     => 0.316227766,
+        MAX    => 0.833333,
+        MEAN   => 0.3923075333,
+        SD     => 0.3162277347,
     );
 
     my $stats = $mx->get_summary_stats;