Rename FunctionBase to Routine

FunctionBase is a clunky name and hard to use in comments/documentation (e.g.: we currently say things like `foo accepts a function/proc/block` rather than `foo accepts a FunctionBase` because we don't want comments to be too tightly coupled to non-user friendly names). We could rename FunctionBase to Routine, and then explain in the documentation that functions/procs/blocks are all types of routines. A function/proc can then be referred to as a schedulable routine.