making use of pypact reading for density

[shimwell]

Recently I added the ability for pypact to read the density keyword from a fispact input file
fispact/pypact#59

we can now make use of this in the script and save some lines of code

this code block

def read_density(filepath):
     lines = open(filepath).readlines()
     for line in lines:
         spl = line.strip().split()
         if 'DENSITY' in line:
             assert('DENSITY' == spl[0])
             return float(spl[1])
     raise ValueError('Density not found')

can be replaced by this

def read_density(filepath):
     ff = pp.InputData()
     pp.from_file(ff, filepath)
     return ff._density