Datashader aggregation function can't be specified by user #314

So far, sum() is the default, but we should include also any, count, m2, mean, mode, std, sum, var.

Complete list: https://datashader.org/api.html#reductions
@timtreis and I excluded some because they don't make sense in our situation or are too complicated to implement.

• use sum as default aggregation
• add other possibilities as kwarg so that the user can specify if they want a different aggregation
• document