Couple More Fixes

I noticed NH wasn't entirely enabled for SPH use in the Toolbar code; added the additional SPH flag. Found an exception after using NH on a part, then deselecting, reselecting and continuing work on the same node would cause an ArgumentException in this._nodePosBackup.Add(): I think I fixed this correctly, but I only followed the code enough to find the most obvious possible fix. Finally, I tried setting the step width to 0.00005 and the original _parseStepWidth() and _parsePlaneRadius() were using ToString() without format specifiers - giving "5E-05": I figured your _formatNumberForOutput would be the best way to avoid the scientific notation formatting. (And then I introduced a bug in that, so I fixed that.)

Edit: And then I realized I never actually tested the SPH fix, and derp, it didn't work so I fixed that fix too.