Fixes int overflow when requesting large slices

[jasonb5]

Fix for #383

The large slice size (300, 60, 384, 320) was causing the variable holding the total number of elements to overflow. The variable was type int, whose max value is 2,147,483,647 and the request slice was 2,211,840,000, causing the overflow. I've changed the type to unsigned long which i dont forsee anyone reading that much data into memory anytime soon.

@muryanto1 How would you like to tackle writing a test for this?

[coveralls]

Coverage remained the same at ?% when pulling de80b82 on large_file_overflow_fix into 34cec4f on master.

[durack1]

@jasonb5 @muryanto1 absolute champs, I'll take nightly for a test and let you know if I hit any ongoing problems.

For a test, I am not sure how we can get around having a huge (>8Gb) dataset to read?

[hyma68]

@jasonb5 @muryanto1 @durack1
I just tested the nightly version on NERSC cori use the following command:

conda create -n nightly -c cdat/label/nightly -c conda-forge cdat python=3.6

I still got zero values when loading a large size array. My array size is (56623106, 137), which is 7,757,365,522. This should be within the "unsigned long" limit. Did I get the right nightly version?

[muryanto1]

@hyma68 Please do:
conda create -n cdms_nightly -c cdat/label/nightly -c conda-forge cdms2 python=3.7.
I only built for python 2.7 and python 3.7 https://anaconda.org/cdat/cdms2/files
I hope you can use python 3.7.

[hyma68]

@muryanto1
Thanks! It is working now for loading large size array. I was able to do python 3.7.
Another question, it seems I can't import "cdutil", and "genutil" with this build. Is there a quick solution that I can import other cdat modules including the two with this build? Thanks again.

[muryanto1]

@hyma68 If you need any other cdat packages, you can do:
conda install -n cdms_nightly -c cdat/label/nightly -c conda-forge cdutil genutil
and any other cdat packages you may need.

[durack1]

@muryanto1, if it's not complicated to implement, could this fix be rolled out for all nightlies? It would create more work for you guys to address continuing bug reports when turning on broader nightly builds could solve this long-standing issue