Tracing tool's project statistics: use TH tags for table header
Tracing tool's project statistics: use TH tags for table header

This makes it easier to distinguish between the header and the actual
table data. This change relates to issue #599.

fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,htem/CATMAID,fzadow/CATMAID,htem/CATMAID