Read requested Exodus block-attributes.

Previously, these were not read. This fixes the
`GetCacheOrRead` method to call the exodus routine with
the proper arguments and adds in a place to request them
from the cache when assembling cell-data arrays.

Note that float vs. double arrays are not currently handled properly.

Change-Id: Iac2063d5cfabed7b90aa48b74169e88036724b1c
Test that exodus attributes can be read properly.

Change-Id: Ib1c2cb5fbb740a6a061dd60986460aa4e7b8d863
Merge topic 'exodus-attrib' into master

02452ebb Test that exodus attributes can be read properly.
577db105 Read requested Exodus block-attributes.