Hotfix 1.4.1 - downgrade svglib to avoid missing libcairo on Ubuntu

[rozyczko]

xhtml2pdf has svglib as a dependency. This module doesn't pin the version of pycairo, which in its recent version began to require libcairo to be installed on the Linux host.
We can't require users to install this library BEFORE installing EasyReflectometryLib, so we need to either replace xhtml2pdf or wait for the issue in svglib to be fixed.
Temporarily we can downgrade svglib so the old pycairo is installed.

[rozyczko]

Also, sneakily added a requested minor feature:
if any zero variance is present in the input, disregard the point and notify the user

[damskii9992]

Here, you're filtering out zero-variance values when loading the dataset. I originally though we would just do it when we do the fitting and maybe plot them differently when plotting them to show that they're being filtered out. At least, that is how I plan to do it in EasyImaging.

[rozyczko]

Here, you're filtering out zero-variance values when loading the dataset. I originally though we would just do it when we do the fitting and maybe plot them differently when plotting them to show that they're being filtered out. At least, that is how I plan to do it in EasyImaging.

Hmm yes this makes sense. We still want to have them available, since this is the dataset we loaded.
Only the fitting should be affected, and maybe some resolution functions which rely on variance.

[damskii9992]

Nice, this is also much simpler than the previous way of masking out during loading ^^