feat: [stretch] make crosswalk_spatial check that all columns are numeric

[voglerdaniel]

In working with crosswalk_spatial(), I have noticed that it crashes when any column that is non-numeric is included in the data argument. This makes sense, since you can't weight & estimate non-numerical data. The problem is that this happens after several minutes of calculation. We could save users time by running a very simple check at the beginning of the function and notifying them it will fail immediately.