Add two class attributes

These attributes will track the birth rate of preys and predators during
a season.
Calculate the birth rate of predators

The rate is equal to the number of births during a season divided over
the size of the lattice, the result should be equal to the proportion of
cells not containing a predator that change their state during the
reproduction stage.
Calculate the prey birth rate

The rate is equal to the number of births during a season divided over
the size of the lattice, the result should be equal to the proportion of
cells not containing a prey that change their state during the
reproduction stage
Typecast dividends to obtain a float result

I should have looked this up ages ago!
Keep track of the current stage being processed

Such information is necessary to calculate the correlation between the
size of a population and its birth rate during a season.
Keep track of numbers before reproduction

In order to make a correlation is necessary to know the numbers of preys
and predators before the correspondig reproduction stages.
Modify method of BatchDialog class

The method now keeps track of the birth rate of preys and predators, and
the number of preys and predators before the corresponding reproduction
stage.
Measure the death rates of the populations

Implement the public interface for these data.
Write death rates and corresponding density data
Reset metric counters when initializing the CA
Measure the probability of death of predators

Implent the corresponding public interface.
Output information related to death rates

The information should be written to the results file created by the
batch dialog.
Measure the probability of death of preys

Implement the corresponding public interface.
Add the probability of death of preys to results

The data should be written to the main results file and the files
created by the batch dialog.
Delete death rates

Since the probability of death of preys and predators offer better
results, only these measurements should be kept.
Remove predator count before death stage

It is no longer necessary to count the number of predators before their
death stage, so remove the code related to this process.
Merge branch 'feature/metrics'