Use flags to build geometry instead of relying on experiment name

[leadmocha]

Right now we have a few scattered conditionals that check for some experiment names to decide what to build. But these are scattered across the different source files.

I think we should, instead, make flags that get configured in just one file. Ex:

if (kGEp) {
fBuild48D48 = true;
fBuildTargetSnout = true;
// ...
} else if (kGMn) {
fBuild48D48 = true;
fBuildTargetSnout = false;
// ...
}

That way, all the settings are in one file, and can be re-configured much quicker and hopefully, with less errors.