Diag decimation prototype, requesting in diag_table

- This update allows the use to request a level 2 decimated diagnostics
  in the diag_table as following example shows

OMp5
1900 1 1 0 0 0
"ocean_hour",            0, "days",   1, "days", "time"
"ocean_model",   "tos",          "tos",              "ocean_hour",         "all", "mean", "none",2
"ocean_model",   "thetao",       "thetao",           "ocean_hour",         "all", "mean", "none",2
"ocean_model",   "umo",          "umo",              "ocean_hour",         "all", "mean", "none",2
"ocean_model",   "vmo",          "vmo",              "ocean_hour",         "all", "mean", "none",2
"ocean_model",   "volcello",     "volcello",         "ocean_hour",         "all", "mean", "none",2 # Cell measure for 3d data
"ocean_hour_d2",            0, "days",   1, "days", "time"
"ocean_model_d2",   "tos",          "tos",              "ocean_hour_d2",         "all", "mean", "none",2
"ocean_model_d2",   "thetao",       "thetao",           "ocean_hour_d2",         "all", "mean", "none",2
"ocean_model_d2",   "umo",          "umo",              "ocean_hour_d2",         "all", "mean", "none",2
"ocean_model_d2",   "vmo",          "vmo",              "ocean_hour_d2",         "all", "mean", "none",2
"ocean_model_d2",   "volcello",     "volcello",         "ocean_hour_d2",         "all", "mean", "none",2 # Cell measure for 3d data

- At the moment it works only for "Native" grid diagnostics and level 2 decimation (bination?)
- It has to be extended to non-native diagnostics, e.g.,
"ocean_model_z_d2",   "tos",          "tos",              "ocean_hour_z_d2",         "all", "mean", "none",2

- It has to be extended to arbitrary level of decimation, e.g.,
"ocean_model_z_d4",   "tos",          "tos",              "ocean_hour_z_d4",         "all", "mean", "none",2
"ocean_model_z_d2",   "tos",          "tos",              "ocean_hour_z_d2",         "all", "mean", "none",2

- Also, note that this prototype only works for smart choices of layouts where "combined" cells are on the same pe
  We need a major design revision to extend this to arbitrary layouts that would need halo updates and halo handling.