Improve error message initialisation SpanROI

[ericpre]

Progress of the PR

• Improve error message when initialising ROI with left >= right.
• add tests,
• update changelog,
• ready for review.

Minimal example of the bug fix or the new feature

import hyperspy.api as hs
hs.roi.SpanROI(30, 15)

raises an confusing message:

Traceback (most recent call last):

  File "<ipython-input-1-1e344269bc12>", line 2, in <module>
    hs.roi.SpanROI(30, 15)

  File "/home/eric/Dev/hyperspy/hyperspy/roi.py", line 688, in __init__
    self.left, self.right = left, right

  File "/opt/miniconda3/lib/python3.8/site-packages/traits/trait_notifiers.py", line 346, in __call__
    handle_exception(object, trait_name, old, new)

  File "/opt/miniconda3/lib/python3.8/site-packages/traits/trait_notifiers.py", line 148, in _handle_exception
    raise excp

  File "/opt/miniconda3/lib/python3.8/site-packages/traits/trait_notifiers.py", line 340, in __call__
    self.handler(*args)

  File "/home/eric/Dev/hyperspy/hyperspy/roi.py", line 703, in _right_changed
    self.right = old

  File "/opt/miniconda3/lib/python3.8/site-packages/traits/trait_notifiers.py", line 346, in __call__
    handle_exception(object, trait_name, old, new)

  File "/opt/miniconda3/lib/python3.8/site-packages/traits/trait_notifiers.py", line 148, in _handle_exception
    raise excp

  File "/opt/miniconda3/lib/python3.8/site-packages/traits/trait_notifiers.py", line 340, in __call__
    self.handler(*args)

  File "/home/eric/Dev/hyperspy/hyperspy/roi.py", line 702, in _right_changed
    self.left is not t.Undefined and new <= self.left:

TypeError: '<=' not supported between instances of '_Undefined' and 'float'

With this PR:

Traceback (most recent call last):

  File "<ipython-input-1-1e344269bc12>", line 2, in <module>
    hs.roi.SpanROI(30, 15)

  File "/home/eric/Dev/hyperspy/hyperspy/roi.py", line 689, in __init__
    raise ValueError("`left` must be smaller than `right`.")

ValueError: `left` must be smaller than `right`.

[codecov]

Codecov Report

Merging #2604 (93b9feb) into RELEASE_next_patch (8394ef0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@                 Coverage Diff                 @@
##           RELEASE_next_patch    #2604   +/-   ##
===================================================
  Coverage               76.44%   76.44%           
===================================================
  Files                     202      202           
  Lines                   29686    29688    +2     
  Branches                 6492     6493    +1     
===================================================
+ Hits                    22694    22696    +2     
  Misses                   5223     5223           
  Partials                 1769     1769           

Impacted Files	Coverage Δ
hyperspy/ui_registry.py	41.66% <ø> (ø)
hyperspy/utils/__init__.py	100.00% <ø> (ø)
hyperspy/misc/label_position.py	86.55% <100.00%> (ø)
hyperspy/roi.py	77.34% <100.00%> (+0.07%)	⬆️

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[ericpre]

@magnunor, @thomasaarholt, this PR is simple, can one of you two please review it?

[magnunor]

Nice improvement! Good error messages are important.

Some comments in the code.

[ericpre]

All done with a a few deprecation warning fixes.