Jedwards/parallel gdal

[jedwards4b]

Potentially replaces #2018 Currently debugging tests.

[jedwards4b]

@msl3v this branch includes the GDAL option in the cmake build (but not yet in the autotools build).
It passes all of the tests except the test_gdal.c (with and without -DPIO_USE_GDAL=ON -DGDAL_ROOT=/path/to/gdal/root)
I don't think that you ever had a working gdal test - did you? Also only one datatype, PIO_DOUBLE, is supported for reading gdal - is that correct?

[jedwards4b]

Update - the autotools build now works and all non-gdal related tests are passing.

[msl3v]

Hey @jedwards4b. Couple of things:

1. It looks like this is the current working PR, correct?
2. Remind me where the code is limited to reading PIO_DOUBLE, please. Where I thought to look I see double, float and int reads. But there may be some upstream code that limits the read to PIO_DOUBLE (it's been a while)
3. You're correct, there was never a gdal_test. I simply used the ESMF test harnesses for developing this.