plot_spectra heatmap should take kwarg as well

[HanHsuanWu]

As title, currently plot_spectra heatmap does not take kwarg for .plot().

[ericpre]

The docstrings on **kwargs needs to be updated, for now, it says "Has no effect on 'heatmap' style."
It also needs a changelog entry - this is useful to inform user of change of behaviour but also for developer to keep track when changes have implemented.

Can you rebase the changes on the RELEASE_next_major branch, because we are aiming at releasing hyperspy 2.0 mid-Sept. and working on that branch will avoid us to merge the RELEASE_next_minor into the RELEASE_next_major.

[HanHsuanWu]

Hi Eric,

I change the doc for plot_spectra and added a changelog entry.
I have rebased to next_major and changed the commit to next_major.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c0f06a3) 81.31% compared to head (1e3f129) 81.31%.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           RELEASE_next_major    #3219   +/-   ##
===================================================
  Coverage               81.31%   81.31%           
===================================================
  Files                     173      173           
  Lines                   24206    24206           
  Branches                 5623     5623           
===================================================
  Hits                    19683    19683           
  Misses                   3223     3223           
  Partials                 1300     1300           

Files Changed	Coverage Δ
hyperspy/drawing/utils.py	76.56% <100.00%> (ø)

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[ericpre]

Suggested clarifications/more details for the docstring.