Refactor invariant perspective

[butlerpd]

At present the invariant calculation has several limitations, including requiring a knowledge of the contrast term from which it computes the volume fraction but not the other way round. Other problems include lack of input parameter uncertainty propagation and the immediate computation of the (incorrect) invariant as soon as the perspective is called.

TODO task list for this issue:

• [ ] Refactor the Invariant to accept either the vol fraction or the contrast factor as inputs and compute the other using the invariant.
• [ ] Accept uncertainty values along with the parameter itself (Porod, contrast, vol fraction) and propagate the through to the uncertainty on computed values. Currently, only the uncertainty on intensity is all that is propagates.
• [ ] Do not supply default values for automatic computation of the Invariant. Currently the contrast factor defaults to 1e-6 and an incorrect invariant is immediately calculated and displayed in the GUI.
• [ ] Edit documentation to match changes

[ajj]

Propose to also refactor the GUI to put input parameters on the main tab - usability wise, if we require the user to input vol fraction or contrast (no default) then we should have that on the first thing they see rather than them having to know to click on the options tab.

Also suggest that we automatically pop up the status window when a calculation is done - that info is rather critical for this calculation.