Provide a way to use only IAU1980 or IAU2000A information

[ronisbr]

Hi @helgee ,

Is it possible to modify the package so that we can work only with the IAU1980 or IAU2000A file separately? I mean, if you are using the FK5 theory, then you will not need to download the IAU2000A files.

Moreover, this would also be interesting for offline files. I mean, if I downloaded and stored the IAU1980 file somewhere and I am using only FK5 theory, then it would be nice if I can pass the location to that file only.

[helgee]

Makes sense 👍 I will have a look, should not be too difficult.