make Parameter.vary a property

[newville]

This makes Parameter.vary into a property, wrapping an underlying ._vary attribute, so that setting Parameter.vary=True will erase an existing constraint expression

This addresses the problem described in #859

Description

Type of Changes

• Bug fix
• New feature
• Refactoring / maintenance
• Documentation / examples

Tested on

Python: 3.11.0 | packaged by conda-forge | (main, Jan 15 2023, 05:44:48) [Clang 14.0.6 ]

lmfit: 1.2.0rc1.post0+g52012fd.d20230405, scipy: 1.10.1, numpy: 1.24.2, asteval: 0.9.29, uncertainties: 3.1.7

Verification

Have you

• included docstrings that follow PEP 257?

• referenced existing Issue and/or provided relevant link to mailing list?

• verified that existing tests pass locally?

• verified that the documentation builds locally?

• squashed/minimized your commits and written descriptive commit messages?

• added or updated existing tests to cover the changes?
• updated the documentation and/or added an entry to the release notes (doc/whatsnew.rst)?
• added an example?

[codecov]

Codecov Report

Merging #860 (8e5c5d2) into master (52012fd) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #860      +/-   ##
==========================================
+ Coverage   93.53%   93.54%   +0.01%     
==========================================
  Files          10       10              
  Lines        3602     3610       +8     
==========================================
+ Hits         3369     3377       +8     
  Misses        233      233              

Impacted Files	Coverage Δ
lmfit/parameter.py	99.03% <100.00%> (+0.01%)	⬆️

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