move away from fully generic API for evald

The use of `xexpression` and template parameter was showing its limits
as the data type could not be different between the different n-dim
inputs (e.g. masks had to be as double rather than bool). Since there is
no real need to be able to use `evald` on `xarray`, the genericity of
its API is reduced so that it now expects inputs to be tensor only. The
number of dimensions for the tensors becomes a template parameter.