Error if first element of a NonParametric generation interval PMF is non-zero

[sbfnk]

          If `NonParametric` generation interval PMFs are going to be zero-indexed, I suggest erroring whenever there is non-zero mass in the 0 bin. I think this will cause the fewest (and the least costly) surprises for users.

Originally posted by @dylanhmorris in #799 (comment)

[sbfnk]

But if users might want this they could just set the first element to zero?

If what you mean is to go through a deprecation cycle for this I agree.

[seabbs]

I changed my mind but yes agree on a depreciation cycle