Allowing to define an array varible with component size one

[yjung-anl]

Reason

An array variable is automatically defined as an normal variable if its component size is one.
This is a reasonable implementation in for most of cases.

In the HFEM-PN solver of Griffin, the solution is expnanded using the spherical harmonics functions. Its expansion coefficients are dofs of the HFEM-PN solver. The expansion coefficients are defined as array variables and the array kernels for HFEM-PN equations were developed. The number of components in array variables are determined the expansion order of spherical harmonics. If the expansion order is set one, the number of component of array variable become one.Then, this variable is automatically defined as normal variables, which are incompatible with the array kernels.

In order to make it compatible with the array kernel in any expansion orders, a new option for defining an array varialbe regardless of its component sizes (allow us to define an array variable with size one).

Design

A new input parameter, is_av, was added to AddVariableAction::determineType in AddVariableAction.C:

std::string
AddVariableAction::determineType(const FEType & fe_type, unsigned int components, bool is_fv, bool is_av)
{
  if (is_fv)
    return "MooseVariableFVReal";

  if (components > 1 || is_av)
  {
    if (fe_type.family == LAGRANGE_VEC || fe_type.family == NEDELEC_ONE ||
        fe_type.family == MONOMIAL_VEC)
      mooseError("Vector finite element families do not currently have ArrayVariable support");
    else
      return "ArrayMooseVariable";
  }
  ...
}

Some minor changes were made accordingly to MooseVariableBase::MooseVariableBase and SystemBase::addVariable.

Impact

A new option become avaiable which allows to define a array variable with component size one.

[yjung-anl]

@YaqiWang
I created an issue for array variable with component size one.