Add context manager to switch iterpath

[ericpre]

Progress of the PR

• Add context manager to set switch iterpath,
• Remove _iterate_signal of lazy signal, its non-lazy counterpart works fine
• simplify _iterate_signal and make it use __call__ so that lazy signal would benefit from the speed improvement in Improve plotting of lazy signals, by keeping current chunk #2568,
• update docstring (if appropriate),
• update user guide (if appropriate),
• Fix warning when building user guide,
• add an changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting changelog entry in the readthedocs doc build of this PR (link in github checks)
• add tests,
• ready for review.

Minimal example of the bug fix or the new feature

import hyperspy.api as hs
import numpy as np

s = hs.signals.Signal1D(np.arange(2*3*4).reshape([3, 2, 4]))

with s.axes_manager.switch_iterpath('serpentine'):
    for indices in s.axes_manager:
        print(indices)

[codecov]

Codecov Report

Merging #2795 (2bb9976) into RELEASE_next_minor (1eb0bdf) will decrease coverage by 0.00%.
The diff coverage is 96.29%.

@@                  Coverage Diff                   @@
##           RELEASE_next_minor    #2795      +/-   ##
======================================================
- Coverage               78.44%   78.43%   -0.01%     
======================================================
  Files                     203      203              
  Lines                   31269    31253      -16     
  Branches                 6861     6857       -4     
======================================================
- Hits                    24528    24514      -14     
+ Misses                   4980     4978       -2     
  Partials                 1761     1761              

Impacted Files	Coverage Δ
hyperspy/_signals/lazy.py	91.65% <ø> (+0.29%)	⬆️
hyperspy/datasets/artificial_data.py	98.64% <ø> (ø)
hyperspy/signal.py	75.92% <90.00%> (-0.20%)	⬇️
hyperspy/_signals/signal1d.py	77.77% <100.00%> (+0.04%)	⬆️
hyperspy/axes.py	90.59% <100.00%> (+0.08%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1eb0bdf...2bb9976. Read the comment docs.

[thomasaarholt]

Lots of nice things here!

LGTM!

[thomasaarholt]

What is "the signal iterator" that this is faster than? Assuming you mean for si in s?
How about """Iterates over the signal data. It is faster than iterating over a signal, since it returns the array directly and not a HyperSpy signal.

[ericpre]

Actually, it would be useful to explain why it is faster! I don't remember right now and I will need to look it up again.

[thomasaarholt]

Yes, that was my best guess :p

[ericpre]

What is "the signal iterator" that this is faster than?

The BaseSignal class has an iterator implementation, i. e. __iter__ and __next__ are implemented, see https://docs.python.org/3/library/stdtypes.html#iterator-types for more explanation.

[ericpre]

@thomasaarholt, I have improved the docstring, can you please have a look at it again?

[thomasaarholt]

Yep, that looks good and is sensible! You can merge when you want :)