FIX - wcst_import should convert number to datetime string

[Bang Pham Huu]

for example:

              "axes": {
                        "time": {
                            "min": "2015",
                            "irregular": true,
                            "dataBound": false,
                            "areasOfValidity": [
                                {
                                    "start": "2015",
                                    "end": "2025"
                                }
                            ]
                        },

here time is AnsiDate axis and "min": "2015", should be converted to "2015-01-01" implicitly instead of 2015 seconds and set the min value to time("1970-01-01T00:33:35 00:00").

Currently, one has to do that manually in the ingredients file with:

                       "time": {
                            "statements": "from datetime import datetime",                        
                            "min": "datetime.strptime('2015', '%Y').isoformat()",