Make erfa source directory a constant in erfa_generator

[eerovaher]

Currently erfa_generator accepts the location of the erfa source files both as a command line argument and as an argument for its main() function, with the default being the correct directory in the liberfa submodule of this repository. To me this looks like a leftover from the time before erfa was included in pyerfa as a submodule and I don't think this functionality is useful. It is much simpler if the location of the erfa source files is specified by a module level constant in erfa_generator.

This can break backwards compatibility for those running erfa_generator directly as a script and for those who import erfa_generator and call its main() function directly, but I suspect I am the only person who runs the erfa_generator script directly and that no-one at all is importing erfa_generator.

[eerovaher]

ERFA_SRC is the name already used by setup.py.

[avalentino]

Sorry but I don't see a big advantage in removing the srcdir parameter.
I agree that erfa_fenerator.py maybe is not used very often but but still IMHO it is a good idea to have its inputs as parameters and configurable configurable via CLI.

Would it be a big issue for you to keep it?

[eerovaher]

I don't really like that it is possible to customize the location of the erfa source files if erfa_generator is invoked directly, but not when pyerfa is being installed, and removing functionality from erfa_generator is simpler than adding functionality to setup.py. But if you really think that this should be configurable from the CLI then close this PR without merging and I will open a new PR that allows configuring the erfa directory when installing.

[avalentino]

Oh, maybe I made some confusion.
What I meant is that the erfa_generator.py should be configurable form CLI so I would not remove the feature from it.
My idea is that someone could need to run it offline to troubleshooting or to develop new functionalities.

Regarding the setup.py script , I agree with you that additional configurability is not needed.
We could even invoke the erfa_generator.py without parameters from setup.py.
The rationale is that the wrapper generation only happens when we generate the packages and it is OK to use the erfa C code in the submodule for that.

[eerovaher]

Does the location of the templates also have to stay customizable from the CLI?

[avalentino]

Yes, to me it makes sense