Verify that inner rotor can reduce down to simpler geometry

[elsevers]

Please verify that the features added in PR #34 do not create duplicate lines when the parameters indicate that aspects of the geometry are not present. For example, consider the following edge cases:

• when the iron tooth height is such that the iron poles vanish between magnet segments
• when only one magnet segment is present
• when the iron tooth span between magnets is zero
• when the intrapolar iron tooth span is zero (iron tooth between poles)

Further:

• Identify all corner conditions that must be tested to insure valid geometry (i.e., make sure that a_rm can never be larger than 180 degrees)
• Implement error checking within the crossSect class
• Create unit tests each case and place in the tests folder.

[Nick-Hemenway]

Check edge case of R_ri going to zero. It should become a solid rotor.

[ngadiyar93]

Verified that R_ri = 0 does lead to a solid rotor and implemented following checks to ensure valid geometry..

1. alpha_rs (segment span) < alpha_rm (pole span) for s (no. of segments)>1
2. d_rs = 0 if s=1
3. d_rs > 0 if s>1
   Working on identifying and implementing tests for rest of the corner conditions.

[npetersen2]

@ngadiyar93 have you added some tests for this?