Invariant failures

[butlerpd]

Describe the bug
A couple of major issues were found during code camp:

1. the q range is no longer settable. The range test boxes act as labels and the range sliders do not update the boxes either.
2. Using the example apoferritin.txt data, the perspective computes ok with the default values for background(0) and scale (1) and contrast (8e-6). It also works if you choose to extrapolate both low and high Q. However, if one enters correct values (from the fitting of the data), namely 0.0023 background, 0.0003 scale and 4e04 SLD the calculator fails with:Calculation failed: not enough values to unpack (expected 3, got 0). Enabling the extrapolations leads the same error being thrown for each of the extrapolations (so 3 errors). E.g. the low Q extrapolation leads to Low-q calculation failed: not enough values to unpack (expected 3,got 0)

It is not clear why the correct values should fail unless it is due to the large error bars at high q which cannot be removed due to inability to constrain the q range used.

To Reproduce
Steps to reproduce the behavior:

1. Load the apoferritin.txt example data
2. send the data to the invariant perspective -- things should work
3. try to adjust the range (either with the sliders or manually in the panel) -- you cannot
4. Change the input parameters to reasonable values
5. the computation now fails

Expected behavior
it should work

SasView version (please complete the following information):

• Version: 5.0.5

Operating system (please complete the following information):

• OS: [e.g. Windows 10]

[krzywon]

#2357 only addressed part of the issue - reopening.