Small Bugfix: Fix an issue outputting >4GB single variables per rank

[bprather]

parthenon_hdf5.cpp allocates a std::vector cache of the largest per-block variable size times the number of blocks. However, both numbers are declared as ints, so when they are multiplied it overflows when outputting a variable of total size >4GB from a single MPI rank.

Oddly, this raises the following tangentially-related error:

terminate called after throwing an instance of 'std::length_error'
  what():  cannot create std::vector larger than max_size()

There are various ways of fixing this, but I chose the easy one.

PR Checklist

• Code passes cpplint
• New features are documented.
• Adds a test for any bugs fixed. Adds tests for new features.
• Code is formatted
• Changes are summarized in CHANGELOG.md
• Change is breaking (API, behavior, ...)
  • Change is additionally added to CHANGELOG.md in the breaking section
  • PR is marked as breaking
  • Short summary API changes at the top of the PR (plus optionally with an automated update/fix script)
• CI has been triggered on Darwin for performance regression tests.
• Docs build
• (@lanl.gov employees) Update copyright on changed files

[lroberts36]

LGTM