Make IPython and IParallel optional

[ericpre]

Progress of the PR

• Make ipyparallel and ipython optional dependencies,
• Fix behaviour of some print_* functions, which were not printing when running from a script, they were only printing when running in notebook or qtconsole. Now all print_* functions behave consistently: they all print the output and doesn't return an object, which was an str or html. The ipython display will pick the correct rendering when running in an "ipython" context, for example notebook, qtconsole.
• Remove fancy to some print_* functions, which wasn't doing anything,
• Bump prettytable to 2.3 to simplify code,
• update docstring (if appropriate),
• update user guide (if appropriate),
• 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

To test the print_* funtions:

import hyperspy.api as hs

hs.print_known_signal_types()

s = hs.datasets.example_signals.EDS_TEM_Spectrum()
m = s.create_model()
m.print_current_values()

[codecov]

Codecov Report

Patch coverage: 92.00% and project coverage change: +0.12 🎉

Comparison is base (cc03d73) 81.23% compared to head (abd7004) 81.36%.

Additional details and impacted files

@@                  Coverage Diff                   @@
##           RELEASE_next_major    #3145      +/-   ##
======================================================
+ Coverage               81.23%   81.36%   +0.12%     
======================================================
  Files                     176      176              
  Lines                   24409    24406       -3     
  Branches                 5683     5681       -2     
======================================================
+ Hits                    19829    19857      +28     
+ Misses                   3274     3244      -30     
+ Partials                 1306     1305       -1     

Impacted Files	Coverage Δ
hyperspy/misc/ipython_tools.py	16.36% <0.00%> (+1.36%)	⬆️
hyperspy/utils/parallel_pool.py	83.33% <88.00%> (+22.46%)	⬆️
hyperspy/_signals/eels.py	77.61% <100.00%> (ø)
hyperspy/component.py	90.52% <100.00%> (-0.05%)	⬇️
hyperspy/misc/model_tools.py	95.12% <100.00%> (ø)
hyperspy/misc/utils.py	91.49% <100.00%> (-0.02%)	⬇️
hyperspy/model.py	86.69% <100.00%> (-0.05%)	⬇️
hyperspy/utils/__init__.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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