Implement remaining Plot widget changes

[abigailalexander]

While most of the suggested changes to the Plot widget PR have been made, there are still some outstanding changes not yet implemented. These are:

• Possibly extract part of opiParser.ts function parseMultipleNamedProps to make it reusable for other functions
• Be more precise for types of obj and props in parseMultipleNamedProps, rather than using any
• Consider whether we want to apply the same limit on maximum number of traces (20) as CSStudio does
• Currently only support plotting waveform PVs - we want to be able to plot time series data - this involves getting the data concatenation working as well as modify the way incoming data is handled
• Decide whether to use test() or it() for unit tests and enforce this change (this is not necessarily related to the plot widget specifically, but worth mentioning)
• Add tests for xyPlot.ts

Hopefully by implementing these changes, the Plot widget would be complete.

[aawdls]

Please assess h5web and davidia as these are the basis for components used by other groups