ephemerides finding improvement

Hi @GregAshton , I was trying to completely isolate a PyFstat installation inside a venv without relying on anything in my home directory, and so I came up with this proposed solution to get ephemerides paths from the $LALPULSAR_DATADIR env variable. In principle this could be made smarter about which version of the files it chooses, but for now I've just told it to try the new 00-40-DE421 and old 00-19-DE421 standards, then fall back to asking for manual user input. Any other use case can still be covered by the conf file approach, which is still checked first and hence can override the env variable. I also added an error catching for the case where the file exists, but is missing one or both of the actual path definitions, and updated the README.

Do you think this covers all relevant cases?