just like for macros that start and end with `__`, `clang++
-Weverything` likes to complain about macros which start and end with
a single underscore. This is basically the same issue as
Ensembles/ert#1048
- use a opm-macro to reduce code duplication
- add a 'test-suite' target which builds tests. for use if BUILD_TESTING
is 0.
- add a 'check' target which builds tests, then executes them
this is just the result of
```
find -iname "*.[ch]pp" | xargs sed -i "s/ *$//"
find opm/parser/share/keywords -type f | xargs sed -i "s/ *$//"
```
so if it causes conflicts with other patches, the others should get
priority. The rationale behind this patch is that some people tell
their editor to remove white space which leads to larger than
necessary patches...
The class Value<T> will keep track of a named variable, and whether that
particular value has been explicitly initialized. Will throw
std::logic_error if trying to use an unitialized value.
