use static class methods to wrap calls to singleton

[byronwatt]

so that CmdLineOptions::instanceGet()->ParseOptions() is just CmdLineOptions::ParseOptions()

also what is the normal

[byronwatt]

aside:

This would probably fix the 87% coverage of:

https://app.codecov.io/gh/byronwatt/CmdLineOptions/blob/main/example/example_with_error_message.cpp

which looks kinda strange, but perhaps GetInstance is an inline function that has a branch for first/second time called ?