Increase the test coverage

97-98% coverage should be possible (now around 93%).

Tasks:

1. Identify the largest uncovered code sections
2. Write tests - unit tests; regression tests (not very efficient to increase coverage)
3. Consider other measures:

• code removal from python modules
• code refactoring eliminating the non-covered code
• exclude module from coverage reports (not recommended, only where testing, removal, refactoring cannot help)

Here a snapshot of the modules with lower than 85% coverage:

virtmat/language/kernel/__main__.py                     3      3     0%   1-4
virtmat/language/kernel/install.py                     31     31     0%   2-53
virtmat/language/kernel/kernel.py                      57     57     0%   4-101
virtmat/language/kernel/__init__.py                     2      2     0%   4-6
virtmat/language/utilities/ase_handlers.py            118     67    43%   20-32, 37-43, 48-51, 56-59, 64-69, 85-97, 102-112, 117-125
virtmat/language/metamodel/amml.py                     55     17    69%   9-10, 28-29, 35-36, 40-42, 44-46, 48-49, 61-63
virtmat/language/utilities/compatibility.py            25      7    72%   19, 28-31, 36-38
virtmat/language/utilities/units.py                    63     13    79%   22, 27, 34-40, 53-54, 62, 106
virtmat/language/constraints/units.py                  15      3    80%   12, 23-24
virtmat/language/utilities/ioops.py                   116     23    80%   24-26, 42-45, 70, 82, 110, 112-113, 123-125, 131-132, 139, 146-147, 153-156
virtmat/language/utilities/warnings.py                 12      2    83%   15-16
virtmat/language/utilities/lists.py                    24      4    83%   6-8, 16
virtmat/language/utilities/errors.py                  216     34    84%   33-34, 190, 248, 252, 254, 256, 258, 260, 262, 266, 268, 270, 272, 274, 278, 284, 286, 288, 292, 308-311, 313-314, 316-317, 322-323, 328-329, 337-338
virtmat/language/metamodel/view.py                    129     21    84%   57-58, 60-61, 63-64, 71-72, 110, 118-120, 128-131, 134, 152-154, 156