Enhancements in AbstractKernel for scalar problems

[shishiousan]

Following enhancements are planned for scalar problems

Variables

• scalarField_ variables e.g. pressure
• scalarCoefficient for diffusion matrix
• vectorCoefficient for diffusion matrix

Initiate and set method

• scalarCoefficient
• vectorCoefficient

Assemble method

• Diffusion matrix
  • isotropic
  • an-isotropic
• Body Source
• Surface Source
• Point Source

others

• debug for high-oder element