Added support map queryresult groupby

[fmachado]

When the "GROUP BY" clause is used, "tags" are returned as Map<String,String>.
Accordingly with InfluxDB documentation https://docs.influxdata.com/influxdb/v1.2/concepts/glossary/#tag-value, "TAG" is always a String so currently we are not supporting (at)Column annotation on non-String Class fields.
This solves the issue #343.

[codecov-io]

Codecov Report

Merging #345 into master will increase coverage by 0.11%.
The diff coverage is 75%.

@@ Coverage Diff @@## master #345 +/- ##
============================================
+ Coverage 83.33% 83.44% +0.11% - Complexity 197 199 +2 
============================================
Files 16 16 Lines 900 906 +6 Branches 88 91 +3 ============================================
+ Hits 750 756 +6 + Misses 109 107 -2 - Partials 41 43 +2

Impacted Files	Coverage Δ	Complexity Δ
...n/java/org/influxdb/impl/InfluxDBResultMapper.java	87.59% <75%> (-1.03%)	45 <0> (+2)
src/main/java/org/influxdb/dto/QueryResult.java	96.29% <0%> (+3.7%)	6% <0%> (ø)	⬇️

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[majst01]

@fmachado would you mind adding a Changelog entry and extended documentation ?

btw: thanks for this.

[bukajsytlos]

Hello.
We really need use this feature.
I would like to ask when you plan to release this?

Thank you.