Add const char* entry to ADIOS constructors to avoid ugliness

[eisenhauer]

As reported by @khuck , the ADIOS2 C++ API has some unexpected behaviour. In particular, constructors like this:
adios2::ADIOS adios("config.xml");
Do not match with this constructor:
ADIOS(const std::string &configFile, const bool debugMode = true);
Instead, they match with this constructor:
ADIOS(const bool debugMode = true);

Because C++ (at least some compilers) would preferentially convert the "const char*" to "bool", resulting in a config file specification that is silently ignored. For more info see:
https://www.bfilipek.com/2019/07/surprising-conversions-char-bool.html

This PR adds a new entry to the C++ API:
ADIOS(const char *configFile);

And adds a test to make sure that the simple ADIOS("configfile.xml") specification results in an attempt to parse that file, not the specification being ignored. We might want to further examine the ADIOS API for similar instances.