`prospect` displays large numbers in scientific notation

[quietust]

When running prospect all in a suitably large embark region (4x4, 180 Z-levels), the "stone" and "mineral" counters can be large enough that they display in scientific notation.

[DFHack]# prospect all
Base materials:
                     SOIL : 106130
                    STONE : 2.77796e+06
                  FEATURE : 41045
               LAVA_STONE : 15593
                  MINERAL : 1.92594e+06
                    MAGMA : 403110
                     ROOT : 4680
                     TREE : 12107
                 MUSHROOM : 8731

This seems to be a result of matdata storing its tile count in a float, a change made in PR #1774 (before that, it was stored in an unsigned int).