Support slices in read.nc

[mjwoods]

The read.nc function allows start and count arguments to be passed to var.get.nc, but the same arguments are applied to all variables. Thanks to Marc Girondot for reporting this issue.

As a possible improvement, slices could be supported across all variables by specifying dimension names for elements of start and count. Any variables with one or more matching dimension names could be sliced, and all other dimensions could be read in full.