Provide type for x̂, ŷ, ẑ #166
Description
Currently, x̂, ŷ, ẑ are all SVectors.
However, I would like to extend the cross function such that ẑ × could be applied to a vector of 2D-SVectors (i.e., to get the curl and not the gradient as per default of the potential operators).
In order to avoid type piracy, this would require a dedicated type for ẑ (and to be consistent I would create types for x̂ and ŷ).
To backwards compatible one could provide interfaces for all vector operations (+,*, etc). Alternatively, one could use ẑ() to return the corresponding SVector (which would only lead to minimal changes).
I gravitate to the first option.