LCA_oM: Add suport for default computation for B2 and B3 modules based on estimate percentages

#### Description:

The IStructE guidance for computing contributions in the B2 Module for structural elements:
- A default value of 1% of Modules A1–A5 emissions of structural elements may be used, based on RICS and GLA Guidance

And guidance for B3 Module:
- B3 emissions may be estimated as 25% of the relevant Module B2 emissions.

A class to be added that allows for those estimates to be set and computed, either as a global setting or per-element basis.

Should set this up in a way that ensures that this can be exchanged for a more refined way to compute those modules (for the case where more accurate data exists). Might be even more critical for other disciplines, for example cleaning of glass for facades and more complex maintenece/repair procedures for MEP.