class keyword needed before friend
Add warning for GCC under version 4.7.

- lower versions are not supported
Some corrections (mainly typos) in reference manual.
Minor improvements in build and test system.

- ignore *.orig
- *.tst rules in tests directories; to run only one con file
- commented recepie to coberture usage (cover testst)

Conflicts:
	.gitignore
	unit_tests/system/tokenizer_test.cpp
Merge branch 'JS_heat_equation'
Sigma in Darcy flow computed according to Martin, Jaffre, Roberts paper.
- the input key sigma has now meaning of a scaling (dimensionless) parameter, through which one can set crosswind conductivity relative to the tangential conductivity in the fracture
- test con files updated so that the reference results remain unchanged