Export to xlsx - group order - dictionary order

Cross ref issue #12 (closed)

We should consider providing functionality to sort by "group_order", as per the data dictionary, defined in this doc: https://gitlab.com/AGS-DFWG-Web/ASG4/-/blob/master/AGS4/AGS4_1/groupsandheadings4-1.JSON

Extract:

{
"class": "Geotechnical Laboratory Testing (Soils and Rocks)",
"group_description": "Laboratory Vane Tests",
"group_status": "-",
"parent": "",
"child": "-",
"group_order": "94",
"group": "LVAN",
"heading": "LVAN_LAB",
"heading_status": "",
"suggested_unit": "",
"suggested_type": "X",
"description": "Name of testing laboratory/organization",
"example": "ACME Laboratories plc",
"in_group_order": "17"
},