Add Rollup support for hierarchical field #143
Description
The rollup operation is well-suited for the hierarchy fields. As the nested data model, it is very easy to express the rollup record.
{
"2015": {
"Jan" : {
"01" : 53,
"02" : 43,
...
}
},
"2016" : {
"Jan" : {
...
}
}
...
}