188 left window of zero returns an error

[jorpppp]

Closes #188

[jorpppp]

In https://github.com/JMSLab/xtevent/pull/194/commits/f52d1de14114f784c79ffef2c16f1791cb92a1d1 I disabled the smoothest path calculation if the left window is zero.

[jorpppp]

@Constantino-Carreto-Romero I tested for a few more things that could lead to issues with a left window of zero and added some modifications:

• Fix the diff avg error messages in https://github.com/JMSLab/xtevent/pull/194/commits/467c1b960c9e0f9c7d0e925aab51cb982651d347
• Fix the checks for lead selection in the IV case https://github.com/JMSLab/xtevent/pull/194/commits/d0a09f22aecf2bc7074c3ce73b638c263d7d3e29 and https://github.com/JMSLab/xtevent/pull/194/commits/362d1a478c5a92dbce3b7882976ed6643cb0ea93
• Check that there are enough leads for the linear pretrend specification test https://github.com/JMSLab/xtevent/pull/194/commits/1883f807be2abe8424f8f4f860aabd5ac54c5294

I think that's it for possible issues with a left window of zero so I'm merging the PR. We can revisit later if we realize something else is amiss when testing for the release.