Make python component part of build, not install

[AlexanderRichert-NOAA]

Proposing a solution for #520, which is to make the python module build part of the build step, not the install step. This PR rearranges python/CMakeLists.txt to use a new target called "python_mod", with which a custom build command can be run to take care of the setup.py invocation. With these changes, setup.py "installs" into the build space, then that directory is copied to the appropriate location by "make install". I took out the first of the two "python setup.py build" invocations because it didn't seem to be necessary but I can reinstate if needed.

[climbfuji]

I am removing myself from the list of reviewers (but appreciate the heads up), because I don't really know enough about the NCEPLIBS-bufr code.

[climbfuji]

I am removing myself from the list of reviewers (but appreciate the heads up), because I don't really know enough about the NCEPLIBS-bufr code.

Well, I would if I could ;-)

[aerorahul]

I think this is much cleaner and more elegant than the previous solution.
Thanks for the update @AlexanderRichert-NOAA

[aerorahul]

Apologies. I meant to approve, but had the button on "Request changes"

[aerorahul]

I am removing myself from the list of reviewers (but appreciate the heads up), because I don't really know enough about the NCEPLIBS-bufr code.

Well, I would if I could ;-)

boo. there is nothing to know about the bufr code here. its just regular cmake and installation stuff.

[climbfuji]

I am removing myself from the list of reviewers (but appreciate the heads up), because I don't really know enough about the NCEPLIBS-bufr code.

Well, I would if I could ;-)

boo. there is nothing to know about the bufr code here. its just regular cmake and installation stuff.

Just needed an excuse for not having the time ...

[jbathegit]

Thanks Alex and Rahul!