Move gravel ore spawn earlier in biome decoration

Addresses issue https://github.com/mDiyo/TinkersConstruct/issues/32

Moved gravel ore gen to generate after underground vanilla ores, but before any other biome decoration. Result is that gravel ore is now more integrated with terrain gen, covered with snow, etc.

:exclamation: NOTE; Gravel ores now appear more frequently because they are the first thing genned on the surface.  I recommend testing to make sure you are happy with the frequency.