Change default time_dim to 'time' and correct unit tests

As mentioned in issue #35 (closed) and previously discussed in an R user meeting, the time_dim parameter default value, will be changed to 'time' instead of 'ftime'. This change will be a benefit if users use startR::Start to load data and not s2dv::Load because the latter has time_dim fixed as 'ftime' whereas Start is fleixble.

Another change that will take place within this branch is to correct the pipeline. The corrections are also related with the default value of time_dim in unit tests which had some inconsistencies previously.