Allow axis to allocate space for loading TCAs to reduce noise

[jbfaden]

Right now as the TCAs (ephemeris) are loading the DasAxis bounds are smaller than when they are once the data is loaded. This causes problems for software like Autoplot because with the height changing the scientist sees additional changes. For example, the TCAs appear, but the height of the spectrogram also changes. Ideally one thing should change at a time so that people can more easily feel confident in their understanding of the process.

There should be a placeholder where the application developer can specify the number of TCA rows and space is allocated as if they were already loaded. Once the TCA data is loaded, the number of rows used to allocate space should still be used.

[jbfaden]

Note the tcaLabels property could be used for this purpose, but I think sometimes you know roughly how many rows will be sent but not which row is which.

[jbfaden]

The new property tcaRows is added. This will always be used, regardless if a different number of TCA rows are loaded.