Minor change to 'rate' method in LinearSolver.

- 'rate' method in Linear Solver is working famously.
- All types of Neumann _rate BC have been tested and in every one of
them , mass balance is satisfied.
- Neumann flux works too,  but it depends on 'cross_section' property of
the throat which is not available yet but is going to be implemented
soon.
- Some examples for BC have been added to run_script, so the user has no
problem for applying any type of BCs.