updated colors, making a commit to start a PR

[gidden]

Please confirm that this PR has done the following:

• [x] Tests Added
• ~Documentation Added~
• ~Name of contributors Added to AUTHORS.rst~
• [x] Description in RELEASE_NOTES.md Added

Description of PR

This PR finalizes IPCC WG1 color pallet.

Color values correspond to those communicated via private email by the IPCC WG1 TSU and attached here. SSP WGI 2021_corrected.txt

When translated to hex codes with the follow snippet

with open('/Users/matthewgidden/Downloads/SSP WGI 2021_corrected.txt') as f:
    for l in f.readlines()[1:]:
        line = l.strip().split()
        if not line:
            continue
        rgb = np.array([int(x) for x in l.strip().split()[1:]]) / 256
        print(l.strip().split()[0], matplotlib.colors.rgb2hex(rgb))

The following values come out

SSP1 #1e9583
SSP2 #4576be
SSP3 #f11111
SSP4 #e78731
SSP5 #8036a7
ssp119 #00a9cf
ssp126 #003466
ssp245 #f69320
ssp370 #df0000
ssp3low #e61d25
SSP43.4 #2274ae
SSP46.0 #b0724e
SSP53.4OS #92397a
ssp585 #980002
RCP2.6 #003466
RCP4.5 #709fcc
RCP6.0 #c37900
RCP8.5 #980002

The updated test file is attached below

This will close #554

[gidden]

FYI @danielhuppmann @znicholls @mkrapp

[codecov[bot]]

Codecov Report

Merging #566 (9bdf251) into main (32fcd33) will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #566   +/-   ##
=====================================
  Coverage   93.6%   93.6%           
=====================================
  Files         50      50           
  Lines       5313    5313           
=====================================
  Hits        4978    4978           
  Misses       335     335           

Impacted Files	Coverage Δ
pyam/plotting.py	92.8% <ø> (ø)

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[gidden]

Hoping for some comments here from TSU colleagues, otherwise this is ready to merge as and when you need for the release @danielhuppmann

[gidden]

At minimum, I'd ask @znicholls to confirm colors