Excessive output by NLM filter

[rubenhorn]

Issue:
There is WAY too much output being printed to stdout by NLM.cpph without using some debugging flags.

std::cout << "NLM info: [...]

This causes giant output files and excessive runtimes.

Proposed solution:
Restrict this output to (filtering-) debug builds using the corresponding CMake flag/preprocessor macro.
Perhaps something separate from DEBUG_FILTER_PIPELINE should be used.

(Relates to #9)