BUG: negative IALPHAB

[DanielePlacidoPolito]

Describe the bug
Negative value for flag IALPHAB does not work.

To Reproduce
Steps to reproduce the behavior:

1. Go to *operation.xlsx
2. Click on sheets (STR_MIX, STR_SC, STR_STAB)
3. Scroll down to line 8
4. Set the allowed negative value
5. Save and run the software
6. See error

Expected behavior
Magnetic field gradient load from auxiliary input file and exploited in the simulation.

Desktop (please complete the following information):

• OS: Windows 10