Fix linear StepModel, wrong function definition

[matpompili]

Description

lmfit.models.StepModel(form="linear") does not match the definition given in the documentation. The other step functions are centered around center, while linear, as of now, starts at center.
This PR fixes the function definition.

This example code shows the difference:

import lmfit
import numpy as np
import matplotlib.pyplot as plt

model = lmfit.models.StepModel(form="linear")
params = model.make_params()
x = np.linspace(-1, 1)
plt.plot(x, model.eval(params, x=x))
plt.show()

Before the fix:

After the fix:

Type of Changes

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

Tested on

Python: 3.8.10 (default, Nov 26 2021, 20:14:08)
[GCC 9.3.0]

lmfit: 1.0.3.post43+g7381cd2, scipy: 1.8.1, numpy: 1.23.1, asteval: 0.9.27, uncertainties: 3.1.7

Verification

Have you

• included docstrings that follow PEP 257? --> Updated previous docstring

• 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?

[newville]

@matpompili Oh, that's sort of embarrassing -- thanks very much! I think the main change to the code looks fine, and would be willing to merge.

Compared to that, this is a minor point, but I don't quite understand why Pipfile and Pipfile.lock need to be added to .gitignore. Those aren't currently part of the lmfit codebase. I believe (but could be wrong!) that they only come about in certain kinds of environments... I'm not sure why we should care about them. Do we need these?

[matpompili]

@newville they are not needed, but they are files used by environment management programs (like pipenv). I can remove them, but the same argument could go for .idea and .vscode, only used by a fraction of the users.

I'm okay with either option, let me know what you prefer!

[newville]

@matpompili I would say to remove them so that this PR was only about the math of the step function.

[matpompili]

Done! Should be ready now.

[reneeotten]

@matpompili @newville change looks good. Please squash the commits and add an entry in the "whatsnew.rst" file; then it should be good to go.

[matpompili]

@reneeotten @newville Squashed and added to whatsnew.rst

[newville]

@matpompili @reneeotten OK, great, I'll merge. Thanks very much!