[operators.basic] streamline implementation of LincombOperator
[reductors] make ResidualOperator more efficient
[core.cache] make default cache regions configurable via environment
[demos] fix docstring of 'analyze_pickle' demo
[core.logger] allow to disable colors in ColoredFormatter

Up to now this could only be toggled globally.
[core.logger] allow for simultaneous stream and file logging

If the user sets pymor.core.logger.FILENAME, each logger obtained by
getLogger() will additionally log to this specified file. This can be used
to create a log file that should be very close to the output created on the
console, e.g. by:

    from tempfile import NamedTemporaryFile
    import pymor.core
    tmp_filename = NamedTemporaryFile(delete=False).name
    pymor.core.logger.FILENAME = tmp_filename