Extract smooth and non-smooth source functions

[ValentinKaisermayer]

Almost every domain needs sources for the potential and flow variables (voltage/current, pressure/volume flow, temperature/heat flow rate...).

These can be handled with two source components in each domain that interface with source functions from Blocks. A smooth and non-smooth version of each function would also be nice. The smooth versions are already present in Electrical and most of the non-smooth ones are already in Blocks.