Enforce consistent usage of PascalCase in files and classes

[sytuannguyen]

Most Files/Classes follow PascalCase naming convention (e.g. ConstitutiveBase.hpp) but not all. Below is a list of some exceptions

• capillaryPressure/layouts.hpp
• fluid/multiFluidSelector.hpp
• elementFormulations/H1_Hexahedron_Lagrange1_GaussLegendre2.hpp (_ should be removed in this case)
• ...

This is just some examples, but there are many of them. These exceptions should be corrected to make the code more coherent.