Neutcurve

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Fixed
+++++
- Better error message if virus or serum is `NaN`.

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Fixed
++++++
- `CurveFits` now works if `viruses` or `sera` are categorical.

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Added
++++++
- Added `CurveFits.plotViruses` method.

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Fixed
++++++
- Bug fix in ylabel plotting.

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Fixed
+++++
- Better selection of initial fit parameters when `infectivity_or_neutralized` is 'neutralized'.

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Added
+++++
- `infectivity_or_neutralized` option to allow fitting of fraction neutralized as well as fraction infectivity.

Fixed
+++++
- `scipy` deprecation warnings.