Fix concrete material definition as sum of fractions was not 1
Fix concrete material definition as sum of fractions was 0.98 instead of 1 causing DD4hep to produce a warning. It was:
0.520 Oxygen
0.325 Silicon
0.060 Calcium
0.015 Sodium
0.020 Iron
0.040 Aluminium
This patch keeps the same fractions but normalizes them.