Change file reader class allocation methods (ESCOMP#265)

Originator(s): nusbaume

Description (include issue title and the keyword ['closes', 'fixes',
'resolves'] and issue number):

@peverwhee discovered during testing of the changes
atmospheric_physics#258 that two changes needed to be made to the NetCDF
reader class, specifically:

1. The constructor must allocate a pointer, not an allocatable variable
(due to the fact that allocating a variable via a function doesn't have
a Fortran standard-defined order, and so can result in "already
allocated" errors).

2. The CCPP-framework does not currently allow for the passing of
pointers, and so all variables allocated by the reader itself during the
`get_var` method should be of type `allocatable`, not `pointer`. Of
course if we eventually find that we really do need a pointer option
then we can always provide two versions of the class itself for the
developer to choose from.

Finally, this PR contains the addition of a `start` and `count` variable
to use for file data subsetting.

List all namelist files that were added or changed:

List all files eliminated and why:

List all files added and what they do:

List all existing files that have been modified, and describe the
changes:
(Helpful git command: `git diff --name-status
development...<your_branch_name>`)

M       phys_utils/ccpp_io_reader.F90
- Change constructor to pointer, get_var to allocatable, and add
optional start/count variables

M       test/test_schemes/file_io_test.F90
   - Update test scheme to properly use new file reader class

List all automated tests that failed, as well as an explanation for why
they weren't fixed:

Is this an answer-changing PR? If so, is it a new physics package,
algorithm change, tuning change, etc?

If yes to the above question, describe how this code was validated with
the new/modified features:

Author: nusbaume