Update Liquid.pde

The 'contains' function can be simplified so that it is both more efficient and (in my opinion) better readable. If my suggestion is accepted, the related code snippet in Chapter 2 of the book and the file liquid.pde in the folder NOC_2_5_fluidresistance_sequence must be updated as well.
Merge pull request #31 from sch0penhauer/patch-1

Update Liquid.pde