[Feature request] - Implement accurate Build-up Factor (B) compuration

[Radiation-Transport]

Is your feature request related to a problem? Please describe.
For the time being, the Build-up factor (B) is set to 1, thus the shielding estimations, when used, are not conservative for most of the cases.

Describe the solution you'd like
Implement accurate Building Factor (B) computation considering shielding as plane as part of: https://github.com/Fusion4Energy/F4Epurity/blob/developing/src/f4epurity/shielding_estimator.py

Describe alternatives you've considered
Leave as it is.