[WIP] IC/BC/ST tools for FEM

DO NOT MERGE THIS PR YET! 

This PR follows #369 and is aimed at providing the following features required for applying IC,BC1/2/3 and source/sink terms in FEM simulations
- Construction of distributed nodal values from a given spatial function
- Generating boundary mesh elements located on a given geometry (used for BC)
- ?: Integration over specified part of mesh (e.g. boundary integration, integration over parts of a domain)
- some more?

I am pretty sure that @TomFischer did already implement this, although a bit more general (i.e. you could specify an epsilon for maximum distance or something like that ...)