Make SolutionArray formatting consistent for different {fmt} versions

[speth]

Changes proposed in this pull request

• Store pure integer columns as integers
• Be explicit about how to format floating point columns that contain only integers

With {fmt} 6.1.2, this changes the output of:

import cantera as ct
p = ct.Water()
arr = ct.SolutionArray(p, 12)
print(arr)

from

      T        D
0.0 300.0  996.633
1.0 300.0  996.633
2.0 300.0  996.633
3.0 300.0  996.633
4.0 300.0  996.633
..  ...      ...
7.0 300.0  996.633
8.0 300.0  996.633
9.0 300.0  996.633
10.0 300.0  996.633
11.0 300.0  996.633

[12 rows x 2 components; state='TD']

to

      T        D
0   300  996.633
1   300  996.633
2   300  996.633
3   300  996.633
4   300  996.633
..  ...      ...
7   300  996.633
8   300  996.633
9   300  996.633
10  300  996.633
11  300  996.633

[12 rows x 2 components; state='TD']

If applicable, fill in the issue number this pull request is fixing

Closes #1526

If applicable, provide an example illustrating new features this pull request is introducing

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[codecov]

Codecov Report

Merging #1547 (2782780) into main (a8a1c83) will decrease coverage by 0.03%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             main    #1547      +/-   ##
==========================================
- Coverage   70.49%   70.46%   -0.03%     
==========================================
  Files         379      379              
  Lines       59091    59091              
  Branches    21228    21228              
==========================================
- Hits        41658    41641      -17     
- Misses      14353    14369      +16     
- Partials     3080     3081       +1     

Impacted Files	Coverage Δ
src/base/SolutionArray.cpp	78.57% <71.42%> (ø)

... and 3 files with indirect coverage changes

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[ischoegl]

Thanks for taking this on, @speth! ... I definitely appreciate that you were able to identify/resolve the issue.

Caveat.

[ischoegl]

Should we set up ubuntu-multiple-pythons runners to use system_fmt=y, so this is covered by CI?

[ischoegl]

LGTM, thanks!