testing custom_title handling

[melange396]

currently testing with:
https://deploy-preview-79--cmu-delphi-epivis.netlify.app/#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

which encodes:

{"chart":{"viewport":[737451.5,-1471,739209.5,14529],"showPoints":false},"datasets":[{"color":"#39aa9c","title":"value","params":{"xxx":"yyy","_custom_title":"ABCD","_endpoint":"covidcast","data_source":"nchs-mortality","signal":"deaths_allcause_incidence_num","time_type":"week","geo_type":"state","geo_value":"ny"}},{"color":"#434d6b","title":"value","params":{"_endpoint":"covidcast","data_source":"nchs-mortality","signal":"deaths_allcause_incidence_num","time_type":"week","geo_type":"state","geo_value":"pa","_custom_title":"EFGH","custom_title":"IJKL"}}]}

[melange396]

At this point in the execution, d.params no longer includes a number of the keys+values that are passed in via the base64-encoded url string, as you might expect... Its because (as best as i can tell) only the parameters that match function argument names are allowed through the call in loadImpl() that sneakily proxies to an appropriate importXYZendpoint() method. However, _endpoint is added back to the params map where loadDataSet() calls loadEpidata().

The current state of code in this PR might work work if we modify the kludge patchDataSet() above to accept another argument for a custom title that can then be added back into the .params of the DataSet returned (it is already populating the .color attribute). A less hacky way to handle this would be to make the custom title into a proper first-order member of the DataSet class, the way .color and .params are -- but so much of this code is already unsightly, i dont know how much i care about that.