unmask ref pixel in ts2vel and tsview

[bbuzz31]

when the timeseriesResidual.h5 is created with timeseries2velocity.py --save-res, the reference pixel time series is overwritten with np.nan.

Then, when plotting (e.g., tsview.py) the whole dataset is masked out because it gets referenced back to the nan time series. This fix just preserves the 0 vector timeseries at the reference point.

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[yunjunz]

LGTM. Thank you @bbuzz31.