Implement 'Calculator' front-end for Decodables

[igorbonadio]

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[renatocf]

@igorbonadio,

Before we merge it, I'd suggest just one change: change the use of double by the alias Probability. This is a better approach because it has a better semantic meaning, and if we eventually decide to change the type (for float or a multiprecision library), it will be easy to change the entire system. Also, issue #50 will introduce assertions to guarantee all Probability return are within the range [0,1].

I believe to make things even better, we should also introduce an alias Infinity, defined as std::numeric_limits<double>::infinity(). It's not only less verbose but would have the same advantages of the alias Probability. But that's a change for another issue.

[renatocf]

Created: #53. As this is a simple task, we can assign it to Bruno or Ademar, or you yourself can change it quickly if you want so.

[igorbonadio]

I tried to change all probabilities values to Probability type. In fact std::numeric_limits<double>::infinity() is std::numeric_limits<Probability>::infinity() now.

[renatocf]

Perfect. We can merge it. Afterward take a look at my comments in #53. Maybe we should think in a more meaningful type system for values that will differentiate probabilities from logarithm of probabilities.