[Feature request] - Interpolation for buildup factors in shielding calculator

[alexvalentine94]

Is your feature request related to a problem? Please describe.
Currently the buildup factor is set to 1.

Describe the solution you'd like
For thick shields, the buildup factor is required. This involves a 2-way interpolation across the mean free path and energy. The tabulated buildup factors are already available in the code.

Describe alternatives you've considered
Keeping simple approximation.

Additional context

[alexvalentine94]

Documentation is also needed for this.