Fixing Schmidt diffusivity model and adding option of Constant Lewis number

[Cristopher-Morales]

Proposed Changes

Fixing the CONSTANT_SCHMIDT diffusivity model and removing the Constant_SchimdtRANS class because it is not needed. Additionally, the class CUnityLewisDiffusivity has been modified to CConstantLewisDiffusivity in order to allow users to use constant Lewis number for each species but different to one, which is the assumption of Unity_Lewis. 
The option UNITY_LEWIS is still available in order to allow users to use that option. New option, CONSTANT_LEWIS, is added for the purpose aforementioned.
I can add a test case for the additional option if it is needed.

Related Work

This pull request is related to #1762

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