External background simulation

[dguff]

• [x] Write a dedicated generator for external γ and neutrons.
• [x] Implement a sensible biasing
• [x] Implement "water bricks" and possible external absorber coating

[dguff]

External background generator implemented in 5313115cfddea671b4f0e92e3722bb56ac932f43

[dguff]

Major external simulation update (includin importance sampling biasing) in 24eb8c93a686c8a9fa175385a12c30ba6d7d7372

[dguff]

Neutron shielding bricks introduced in 9b9eb1f3a4bf2e136b702c54d29471f77d8d71a5 Updated geometry file for Boulby-size detector in df4ae17b351f34ef5228c7de1b73a78e9d8ce40d

[dguff]

Note: a possible better implementation of the importance sampling may be develop based on example GB05 (Geant4 v11.1.1)