Updates of IoTDB Group By Fill Query

[Mr.Crazy]

Hi!
  I'm working on Group By Fill Query these days, to enrich the 
capabilities of group by fill. Formerly, group by fill only support last_value 
aggregation using PreviousFill. But now group by fill supports all aggregation 
functions and fill methods! I modified the fill syntax and GroupByFillDataSet 
to make this feature work, you can get an overview of the changes below.
  Firstly, our old fill syntax perform Fill by specifying the fill method 
for each type. But specifying multiple fill methods in a single SQL can impose 
a significant logic burden of GroupByFill. So we simplify the fill syntax from 
FILL(<data_type&gt;(fill_method)(, <data_type&gt;(fill_method))*) to 
FILL(fill_method) (on Previous or Linear fill) and FILL(constant) (on Value 
Fill). While the old syntax can be compatible. You should notice that even the 
old fill syntax doesn't allow for multiple fill methods in a single SQL.
&nbsp; You can get more information of fill syntax updates from our UserGuide 
in master branch. Click&nbsp;here&nbsp;for English version.
&nbsp; The GroupByFillDataSet is also rebuilt for supporting new feature. You 
can see&nbsp;this doc for new group by fill examples and get more rebuilt 
details.
&nbsp;&nbsp;
&nbsp; Rongzhao Chen
&nbsp; School of Software, Tsinghua university