Abort if SoftSource is selected for HBias

[RevathiJambunathan]

We need to abort the simulation with a message if soft source excitation is selected for HBias and also clarify this in the documentation

[ajnonaka]

We also need to test for the other excitations (E, B, H) that they are 0, 1, or 2 only once at initialization (they are currently tested every time the hard/soft sources are added).

[ajnonaka]

Implemented in #92