ENH: support exprField specification for SemiImplicitSource

- this allows more flexibility when defining the location or intensity
  of sources.

  For example,

  {
      type            scalarSemiImplicitSource;
      volumeMode      specific;
      selectionMode   all;

      sources
      {
          tracer0
          {
              explicit
              {
                  type       exprField;

                  functions<scalar>
                  {
                      square
                      {
                          type square;
                          scale 0.0025;
                          level 0.0025;
                          frequency 10;
                      }
                  }

                  expression
                  #{
                      (hypot(pos().x() + 0.025, pos().y()) < 0.01)
                    ? fn:square(time())
                    : 0
                  #};
              }
          }
      }
  }

ENH: SemiImplicitSource: handle "sources" with explicit/i...