Allow further arguments in msqrobAggregate()

[cvanderaa]

Currently, msqrobAggregate() does not allow to pass further arguments to aggregateFeatures(), but this would be useful to have better control on the aggregation. For example, one would want to provide an na.rm = TRUE to methods that are susceptible to missing values.

The easiest option is to implement ... in the function signature. It's straightforward and easy to document.