Qfit

None

Enhancement and changes
* Now numerical transition frequencies are always taken absolute value before comparing with experimental data
* Change the display of the mean square error to root mean square error, and the unit is now "MHZ" by default
* For panel to select the data file to display, the data file name will be shown when hovering the cursor on the panel

Bug fixes
* Transitions somehow don't have default names
* Reset button can't really reset x and y tick labels
* Now we allow for the existence of a data file with no extracted data

Enhancements:
* Make the format of the status messages more consistent

Under the hood
* When the window is closed, accessing the internal objects now won't crash the kernel (QFit knows it dies)
* Safer signal connection for QRunnable in QuantumModel class.

Bug fixes:
* Can't choose shgo and differential evolution as the optimizer. Now fixed

New Features
* Now QFit supports the direct control of the window by manipulating `Fit` object. Currently we support
* `Fit.close()`: shut down the app
* `Fit.hide()` and `Fit.show()`: hide and re-show the window whiling keep things running

Bug fixes
* Button `resultToPrefit` does not update the view
* Eliminate warning messages
* Increase the stability when fitting