Neutcurve

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Fixed
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- Fixed bug when IC50 is at lower bound.

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Added
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- Ability to draw vertical lines on neutralization curves (`vlines` option to `CurveFits.plotGrid` and `CurveFits.plotSera`).

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Fixed
+++++
- Better fit curves that never reach IC50.

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Fixed
+++++++
- Fix bug in ymax on some plots generated by `CurveFits`.

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Fixed
++++++++
- Fix bug in `CurveFits.plotGrid` when plotting just wildtype.

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Added
+++++++
- `ignore_serum_virus` to `CurveFits.plotSera`.

- Added options to `CurveFits.plotGrid` to **not** share x- and y-axis, and to allow different labels.