Support single-server view testing

[mcoles]

TabCompare was designed to analyze differences between the same dashboards running on two versions of Tableau Server. However, simply rendering views from a single server to PNG/CSV, reporting errors and loadtimes could be a useful feature in and of itself. Another reason a administrator might want this functionality is for auditing a viz as it changes over time (think the Internet Archive).

It is possible to use TabCompare for these use cases, if you specify the same server as both the --sa and --sb arguments, e.g.:

.\TabCompare.exe --sa 'https://myserver.mycompany.com' --sb 'https://myserver.mycompany.com' --cv --cd --nr 1 --nt 10 --vi c:\TabCompare\MyTop500ViewLuids.csv --u yourusername --f c:\TabCompare\Top500ViewTest

However, this results in double the requests, double the file outputs,and you're still comparing the files, looking for nonexistant differences. The following changes could probably be made to support these kinds of single-server export scenarios:

1. Make --sa optional, but perhaps with a warning that standard comparison functionality will not apply.
2. Skip comparison functions for the single-server scenario
3. Change the .twbx to make it more elegantly support only a single server (maybe a parameter changes the view, or we make a second sheet?). Note that we should continue exporting the same set of columns in the report.csv file.

[mcoles]

So, you can actually already do this if you specify the same server as both the --sa and --sb arguments, e.g.:

.\TabCompare.exe --sa 'https://myserver.mycompany.com' --sb 'https://myserver.mycompany.com' --cv --cd --nr 1 --nt 10 --vi c:\TabCompare\MyTop500ViewLuids.csv --u yourusername --f c:\TabCompare\Top500ViewTest