Use some padding in wflow forcing

[Peter9192]

Currently, no padding is used when extents are inferred from a shapefile:

https://github.com/eWaterCycle/ewatercycle/blob/50e1fa7afd86c574bf74ae2bdffc34226f4e358b/src/ewatercycle/forcing/_wflow.py#L74

In earlier applications we used to take 1.5 deg padding to make sure the forcing extents exceeded the demfile extents. I believe the 1.5 was chosen such that it was a multiple of the grid spacing for era-interim (0.75 deg).

Without padding, I now see some log messages like

2022-07-08 12:32:31,086 [3163697] WARNING  wflow.py,174 Insufficient data available, input data starts at 3.75 degrees longitude, but should be at least one grid cell larger than the DEM start at 3.570833333333352 degrees longitude.

It would be good to add back some padding in the wflow forcing generator