WIP: Spelling fixes in two_power_law model

[llimeht]

Fix typos in variable names in code for two_power_law model.

Marked as WIP as it still needs some the code added so that opening old projects still works.

Closes: #444

[butlerpd]

While a spelling correction, it is a spelling correction to two of the model parameters. Thus this should not be merged until the proper parameter translation is added to the "translation" file or it will break things like a saved project which will be using the old name.

[krzywon]

Copied from #458:

To keep backward compatibility with old save projects saved with old parameter names you will need to update sasmodels/conversion_table.py. The python file houses a dictionary that maps the previous SasView version to the changes made after that version was released. This can accommodate model name and parameter name change.

General usage, but more information is given in the file documentation:

<old_Sasview_version> : {
<new_model_name> : [
<old_model_name> ,
{
<new_param_name_1> : <old_param_name_1>,
...
<new_param_name_n> : <old_param_name_n>
}
]
}

[wpotrzebowski]

@wpotrzebowski to ask @llimeht if he wants to continue working on it