Add Unit Testing for development #56

As the package becomes more and more complex, it is important to adopt unit testing. We can following the example of RMS.jl at https://github.com/ReactionMechanismGenerator/ReactionMechanismSimulator.jl/blob/ac2a3bd88504b86d5bc60dedc09533839a9c9cbb/src/Calculators/TestThermo.jl#L1-L60.

Some thoughts:

• We could put the test files inside the src folder, with a consistent prefix, so that one can easily spot the actual source file.
• We can test the functions against Cantera via PyCall. This should work well for most of the functionality.
• @test Cps ≈ Cpexplist rtol=1e-3 where 1e-3 is a good default value for tolerance.