MSPASS_HOME should not be required

[wangyinz]

Right now, in our C++ code, we rely on the following function to determine the default directory for the parameter files: https://github.com/mspass-team/mspass/blob/bf7e2f05609888b56a238095ff2aa3527e304694/cxx/src/lib/utility/data_directory.cc#L7 Basically, a MSPASS_HOME environment variable needs to be set for any of those C++ code to work.

In the Python code, such requirement is avoided by the following constructs: https://github.com/mspass-team/mspass/blob/bf7e2f05609888b56a238095ff2aa3527e304694/python/mspasspy/db/schema.py#L17-L26 Because we always install the data directory with the Python package, we can use __file__ (which is the pathname of the file from which the running Python module was loaded) to locate the relative path to the data directory .

I should also note that there are Python codes that didn't use this robust method: https://github.com/mspass-team/mspass/blob/bf7e2f05609888b56a238095ff2aa3527e304694/python/mspasspy/preprocessing/css30/datascope.py#L87-L95

Now, I think we should improve the C++ code such that it is not necessary to define MSPASS_HOME especially when it is compiled and installed as a Python modules. This can be done by using CMake to set a variable to tell the code that it is being built as a Python module. Then, CMake can also set a global variable to code in the relative path to the data directory within the python installed directory.

[pavlis]

That makes sense. I'll need some help on how to set up CMake to do that but the C++ change should be simple. I think the MsPASS _HOME functionality should still be retained but is the final fallback if all else fails.

[wangyinz]

I need to further refine this idea. It is tricky to do it right actually. I was just writing down my thought above as a memo.

In any case, I agree the MSPASS_HOME should be kept, but we should just follow the logic of the python code: when MSPASS_HOME is defined, we use that, and if it is not, we try the relative path.