Updates Set methods in ScalarFields 

I think this issue is strongly related the issue number #179  

When a scalarfield is set by the "addContribution=.FALSE." and given "scale" value, 
it does not work properly.

e.g 
```fortran 
CALL obj%set(VALUE=avec,  &
      & addContribution=.FALSE., &
      & scale=alp)
```
it set $`obj = obj + alp*avec`$ instead of $`obj = alp*avec`$.  
