Update step size dtypes

[brandonwillard]

In #86, I updated the step size dtypes to int64, but I imagine it would be better to perform that conversion somewhere within AeHMC; however, I don't know the best place for that at the moment.

@rlouf, if you get a minute, tell me where you think an astype could be performed so that users can specify int32 and avoid those Scan casting issues.

Originally posted by @brandonwillard in #86 (comment)

[rlouf]

As far as I understand, you cast the number of steps to int64. The high-level entry points of the library, and that you might want to use in a Scan are here:

• aehmc/aehmc/window_adaptation.py

  Line 85 in d54e2d0

  def window_adaptation(

• aehmc/aehmc/hmc.py

  Line 73 in d54e2d0

  def step(

It seems that you've covered the internal ones already.