Update integrated flux values automatically when toggling "Integrate Model" on and off

[jbudynk]

If "Integrate Model" is OFF, then you add a passband, toggle the "Integrate Model" button back ON, and add a new passband, the flux value from the new passband is calculated by integrating the model, while the old passband's flux value is still from the raw SED. The user has to click "Calculate" to update the old passband flux value.

Instead, when a user toggles "Integrate Model," the flux values should automatically update.

Test:

SED: 3c066a
model: brokenpowerlaw (default), chi2datavar and neldermead

Integrate model (OFF). Add a user-defined passband from 5000 to 1e5 Angstroms. Add a photometry filter (SLOAN sdss.g).

Turn integrate model ON. Add another user-defined passband from 0.01 to 0.1 keV. The latest passband is calculated from integrating under the model, while the other two passbands still show the results from integrating under the raw SED. Clicking "Calculate" updates the flux values as expected.

[olaurino]

This is related to (and spawned from) issue #80 and PR #105

[olaurino]

This is most likely trivial to fix, but since we had to deal with more pressing issues, I am putting this back into the backlog.