Internally used filter level assignment relied on the name of the modules,
which can be ambiguous. Added an additional check on the type of the module
to be a bit more stable and to prevent from multiple modules on the same level.

The newly introduced function yields the same levels as the previous definition
in dqmBeginRun, without having multiple modules on the same level.

Removed some trailing whitespace as well.

clang complained about arrays of chars not being initialized by a
constant expression. (gcc compiles without warning on this)

Subsequently the call to 'isAnyOf' does not succeed due to:
no known conversion from 'const initializer_list<const char *>' to 'const initializer_list<const const char *>' for 2nd argument

Removing the second const from const char* const would allow clang to compile
but breaks the gcc build due to the same reasoning (but with reversed arguments in this case).

Since this is called only in setup I have simply replaced all constexpr char[] by std::string.

- Avoid usage of stringstream where concatenating strings is possible.
- Make filter level assignment stable enough to allow for early returns.