1.0.4rc1: Merge pull request #831 from lmfit/calc_dely_components

Version 1.0.4rc1

New features:

• add calculation of dely for model components of composite models (Issue #761; PR #826)
• add R^2 rsquared statistic to fit outputs and reports for Model fits (Issue #803; PR #810)
• add SplineModel (PR #804)
• add Pearson4Model (@lellid; PR #800)

Bug fixes/enhancements:

• make sure variable spercent is always defined in params_html_table functions (reported by @MySlientWind; Issue #768, PR #770)
• always initialize the variables success and covar the MinimizerResult (reported by Marc W. Pound; PR #771)
• build package following PEP517/PEP518; use pyproject.toml and setup.cfg; leave setup.py for now (PR #777)
• components used to create a CompositeModel can now have different independent variables (@Julian-Hochhaus; Discussion #787; PR #788))
• fixed function definition for StepModel(form='linear'), was not consistent with the other ones. (@matpompili; PR #794)
• fixed height factor for Gaussian2dModel, was not correct. (@matpompili; PR #795)
• for covariances with negative diagonal elements, we set the covariance to None (PR #813)
• fixed linear mode for RectangleModel (@arunpersaud; Issue #815; PR #816)
• report correct initial values for parameters with bounds (Issue #820; PR #821)
• allow recalculation of confidence intervals (@jagerber48; PR #798)

Deprecations:

• remove support for Python 3.6 which reached EOL on 2021-12-23 (PR #790)