Enable conversion warnings

[joaander]

Description

Enable the compiler option -Wconversion which is not part of -Wall.

Motivation and context

Conversion warnings notify developers when narrowing conversions may result in the loss of data. These will be important to have when we start implementing mixed precision (#355) and combinations of 16, 32, and 64 bit seeds for the RNG (#229). Just in preparing this PR to build warning free I found several bugs in memory allocators, GSD output and other places.

Unfortunately, this PR will touch many files because intended narrowing casts must be explicitly made by the developer.

This option is not part of -Wall so many headers and embedded libraries do not compile warning free with this option. I selectively set -Wno-conversion on those files so we don't need to change them.

How has this been tested?

I'm testing locally with gcc10 and clang11.

Change log

Fixed:

- Enable memory buffers larger than 2-4 GiB.
- Correctly write large image flags to GSD files.
- Support more than 26 default type names.
- Correctly represent fractional degrees of freedom.
- Compute the minimum image in double precision.

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the HOOMD-blue Contributor Agreement.
• My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.