Fix on mechanical index report

• Fix : Correcting Mechanical Index (MI) report in Step 3. We identified that a wrong mask was used to find the highest value of pressure used to report the MI. This error was only applicable to the MI. The fix now uses the same location as used for the maximal Isppa in the brain. Results from studies using versions of BabelBrain prior to 0.4.1 can retrospectively calculate the correct MI using the formula MI=SQRT(Selected_Isppa * 1e4 * 2 * 1041 * 1562)/1e6/SQRT(Frequency_MHz), where Selected_Isppa is the intensity in W/cm2 as used in Step 3 and Frequency_MHz is the selected frequency in MHz.