find_field_indices

'find_file_indices': test field structure for input in proj_field and plot_field
group the variables  into 'fields' with common dimensions
----------------------------------------------------------------------
 function [DimVarIndex,CellVarIndex,NbDim,VarType]=find_field_indices(Data)

OUTPUT:
 CellVaxIndex: cell whose elements are arrays of indices in the list data.ListVarName  
              CellvarIndex{i} represents a set of variables with the same dimensions
 NbDim: array with the length of CellVarIndex, giving its  space dimension
 VarType: cell array of structures with fields
      .coord_x, y, z: indices (in .ListVarname) of variables representing  unstructured coordinates x, y, z 
      .vector_x,_y,_z: indices of variables giving the vector components x, y, z
      .warnflag: index of warnflag
      .errorflag: index of error flag
      .ancillary: indices of ancillary variables
      .image   : B/W image, (behaves like scalar)
      .color : color image, the last index, which is not a coordinate variable, represent the 3 color components rgb
      .discrete: like scalar, but set of data points without continuity, represented as dots in a usual plot, instead of continuous lines otherwise
      .scalar: scalar field (default)