added ValueError for unknown densities

[adriente]

Description of the change

Some entries of the elements database have unknown densities due to their very high radioactivity : Astate and Francium. When calling the function _density_of_mixture from hyperspy.misc.material with these elements it would raise a TypeError with a difficult to understand message.
With this PR, the function raises a ValueError with a more understandable error message.

Progress of the PR

• Change implemented
• ready for review.
• make a test.
• make a changelog entry.

Minimal example of the bug fix or the new feature

import hyperspy.misc.material as m
m._density_of_mixture([1.0],["At"])

New output

Traceback (most recent call last):
  File "/home/teurtrie/hyperspy/hyperspy/misc/material.py", line 224, in _density_of_mixture
    sum_densities[i] = weight / densities[i]
TypeError: unsupported operand type(s) for /: 'float' and 'numpy.str_'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/teurtrie/hyperspy/hyperspy/misc/material.py", line 236, in _density_of_mixture
    raise ValueError(
ValueError: The density of one of the elements is unknown (Probably At or Fr).

[codecov]

Codecov Report

Merging #2775 (fa3e3eb) into RELEASE_next_minor (2ddc359) will increase coverage by 0.30%.
The diff coverage is 93.75%.

❗ Current head fa3e3eb differs from pull request most recent head fcf0fe8. Consider uploading reports for the commit fcf0fe8 to get more accurate results

@@                  Coverage Diff                   @@
##           RELEASE_next_minor    #2775      +/-   ##
======================================================
+ Coverage               78.14%   78.45%   +0.30%     
======================================================
  Files                     203      203              
  Lines                   30681    31281     +600     
  Branches                 6704     6871     +167     
======================================================
+ Hits                    23976    24540     +564     
- Misses                   4963     4978      +15     
- Partials                 1742     1763      +21     

Impacted Files	Coverage Δ
hyperspy/misc/material.py	83.78% <93.75%> (+0.33%)	⬆️
hyperspy/_signals/dielectric_function.py	86.11% <0.00%> (-4.80%)	⬇️
hyperspy/misc/array_tools.py	84.37% <0.00%> (-2.62%)	⬇️
hyperspy/datasets/artificial_data.py	98.64% <0.00%> (-1.36%)	⬇️
hyperspy/_signals/complex_signal.py	93.78% <0.00%> (-1.15%)	⬇️
hyperspy/utils/model_selection.py	81.81% <0.00%> (-0.80%)	⬇️
hyperspy/models/model1d.py	87.96% <0.00%> (-0.71%)	⬇️
hyperspy/misc/utils.py	85.90% <0.00%> (-0.38%)	⬇️
hyperspy/io_plugins/hspy.py	68.93% <0.00%> (-0.37%)	⬇️
hyperspy/signal.py	75.92% <0.00%> (-0.34%)	⬇️
... and 60 more

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[ericpre]

This looks good, it needs a test and to make the PR to the RELEASE_next_patch branch because it is a bug fix.
To avoid pulling change from the RELEASE_next_minor branch, the base branch will need to changed, see https://hyperspy.readthedocs.io/en/stable/dev_guide/git.html#changing-base-branch for explanation. If you prefer, I can sort it out before merging.

[ericpre]

A test is needed to cover this case, you can simply add your example from #2775 in

hyperspy/hyperspy/tests/utils/test_material.py

Line 58 in 2ddc359

def test_density_of_mixture():

.

[ericpre]

It needs a changelog entry in the upcoming_changes folder too - see upcoming_changes/README.rst!

[ericpre]

Thanks, looks good. A few commits are unnecessary and I will remove them before merging.