The aim of this commit is to add new feature for real time tasks simulator
'simso'. We allow WCET of a task to be defined as a discrete Probability
Distribution Function (PDF) using a matrix (numpy array) of two row: one
for possible WCET values and the other for their corresponding
probabilities. When the WCET of a task is defined as probability
distribution, the simulator will pick random value from the possible WCET
values proportionally to their probabilities. Then, it create a new job
that has as WCET the generated random value.

The modified files in this commit are:

    - simso/utils/probabilistic_calc.py: new file contain function that
    generate random value according to a probability distribution.
    - simso/configuration/Configuration.py: In case of probabilistic
    WCET, verify that all WCET values are positive and that
    probabilities sum to one.
    - simso/core/Job.py: define new attribute '_wcet' individual value
    for each job instance.
    - simso/core/Task.py: In case of probabilistic WCET, generate random
    value of WCET using 'randon_int_from_distr' function from
    'probabilistic_calc.py' module. Then, create  new job with
    the generated WCET.
    - simso/core/__init__.py: change import order to avoid program crush.

misc/script.py file is also slightly modified to add a task 'T1' with
probabilistic WCET. When run, this script will generate a scheduling where
several instance of task 'T1' has different WCET values.