Update simulation rml

[mariajmz]

This PR is related to the one I opened (#2) in the alphacamm-simulations repository (before removing the submodules).

Changes that are implemented (this one is not tracked in this repo):

• Updating the <detector> section. Before, we were defining the sensitive volume as <storage>. I think both work but this one is more updated.

Keeping the comment on the old PR, I propose some other changes:

• Generating the alphas by the decay of each isotope. We can configure via rml if we want to simulate the full chain or stop at some point (Add option to stop decay chain at any given isotope rest-for-physics/restG4#132).
• If we want to access to the TRestGeant4PhysicsLists object after the simulation, we couldn't because we are naming it as "default". We can ommit the name part <TRestGeant4PhysicsLists name="default" file="common/physics.xml" />
• Also, a more practical question, do you want to have the rmls divided like they are now or do you prefer to have everything in the same file?