Place dependencies into separate file

[K20shores]

This separate outs the dependencies and partially addresses #2713

• All find_package calls are now in the dependencies.cmake file, making it easyish to see in one place what is needed
• Added a file to start to collect the macros and functions as recommended by the linked issue

[ZedThree]

Probably quite a lot of this could also be moved into separate Find*.cmake files to tidy up even further

[K20shores]

@WardF after @ZedThree's suggestion, the tests pass and it appears this PR is ready to go.

[WardF]

Thanks @K20shores and @ZedThree. Hope everybody stayed/is staying warm. Now that things are thawing out in rural Oregon, I should be able to resume review of the PR backlog.

[WardF]

One last change occurs to me; the new file needs to be added to theEXTRA_DIST definition in the makefile.am file in the corresponding directory. This way, when we create tarballs with make dist (using an autotools-based build), the tarball will have all the files needed for cmake-based builds.

[WardF]

I can make that change; other than that, this looks great, thanks!

[K20shores]

@WardF Makefile.am seems to include the cmake directory; that's where the dependencies.cmake and netcdf_functions_macros.cmake file are. Does it still need to be added to Makefile.am?

[WardF]

If it includes the directory we should be good. I'll double check to be sure, but that's probably got it. Thanks!