Reorganisation and improvements in significant parts of opm-core

Note: DO NOT MERGE until PRs have been made in dependent modules.

Changes include:
 * move opm/core/fluid/ to opm/core/props and subdirs
 * remove unused or irrelevant classes and programs
 * give several classes more precise consistent names
 * make a common interface for (2p incomp) transport solvers, enable run-time flexibility.

Again: DO NOT MERGE yet...