Add comm_code option AG6 as available for HS classification.

[amannj]

Hi,

Option comm_code should ideally also recognise aggregation level AG6 of the HS classification. Please see example below.

Thanks a lot.

---

Example using updated ct_search():

> ct_search(reporters       = "Spain",
+           freq            = "monthly",
+           start_date      = "2020-01", 
+           end_date        = "2020-01",
+           partners        = "China",
+           trade_direction = "exports",
+           commod_codes    = "AG6") %>% 
+   as_tibble() %>%
+   select(classification, period, aggregate_level, trade_flow, reporter, partner, 
+          commodity_code, trade_value_usd, netweight_kg) %>%
+   slice(1:2)
# A tibble: 2 x 9
  classification period aggregate_level trade_flow reporter partner commodity_code trade_value_usd netweight_kg
  <chr>           <int>           <int> <chr>      <chr>    <chr>   <chr>                    <int>        <int>
1 HS             202001               6 Exports    Spain    China   020630                  149985       124614
2 HS             202001               6 Exports    Spain    China   050690                   17720        19104

Data query using comtrade.un.org:

[codecov-io]

Codecov Report

Merging #30 (6dfc20e) into sitc-bug (97c2ee1) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           sitc-bug      #30   +/-   ##
=========================================
  Coverage     77.33%   77.33%           
=========================================
  Files             9        9           
  Lines           472      472           
=========================================
  Hits            365      365           
  Misses          107      107           

Impacted Files	Coverage Δ
R/ct_search.R	85.23% <100.00%> (ø)

---

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[ChrisMuir]

This is great, thanks a bunch! Do you mind pushing a couple more edits to this PR?

• If you'd like, please add your self to the DESCRIPTION file as a contributor. Should look like this.
• Modify the NEWS.md file to add your feature. Something like:

## NEW FEATURES

* Modifications to `ct_search()` to add support for commodity code `ag6` ([#30](https://github.com/ropensci/comtradr/pull/30))

[amannj]

Hi Chris,

Thanks a lot! I've just pushed the edits to the PR.

[ChrisMuir]

lgtm