Reduce Ice-generated C++ warnings

Rather than simply listing (and ignoring) all the warnings
that come with ZeroC-generated code we now suppress their
output at the cmake level.

In addition, using `cmake -Dfatal-warnings=true .`
will make all warnings an error which forces cleaning
up these issues.

/cc @rleigh-dundee @sbesson -- suggestions on better header names welcome.
> As for the other flags below, I'd caution direct use for portability reasons. Also, the set of warnings varies from compiler to compiler (and version to version) so the chance of having a warning-free build across the board for all platforms is fairly low, and the consequent of erroring out makes its utility quite limited. It only takes a new GCC or clang release to add a new warning to the -Wall or -Wextra collections (or increase the strictness of their parser, which happens quite often) for a fatal error to occur. For this flag, the end user can set the compiler-specific option at their own risk, so not sure support for it really belongs in our scripts.

For the `fatal-warnings` flag? If so, it's likely just a question of my not knowing the easiest way to tell `cmake` to add this from the command-line. Suggestions?

> Note that such support is better added to bioformats.git cpp/cmake/CompilerChecks.cmake if we want it in-tree, but it should support all compilers including MSVC and have the appropriate functional checks as we do for the other compiler flags. Happy to do this if you like.

Agreed, but for the moment, I'd propose that we add these in. One library that does something like this is https://github.com/ruslo/leathers but anything that will let me tune up **and** down the warnings would be great.