grid and subcatchment interpolation bug

[barneydobson]

I've had a look at the option to use swmm_stride which lets you control the timesteps more carefully in pyswmm, but, while it changes the reporting to be precise and uniform, it seems to produce confusing simulation datapoints (at least in my initial test). As far as I can tell it is more intended for custom control structures and not simulation reporting. I will leave this for now - though in checking it has revealed a bug in the interpolation code, so still helpful ;). Possibly when swmmanywhere is made open source, if we explore some collaboration with the pyswmm people I am sure they will have a better idea for how to pre-specify reporting timesteps.

Originally posted by @barneydobson in #109 (comment)