Discourage DEFINE NUM_CPU

It has been observed that some users have

`DEFINE 4`

in their Ert configs. This is probably a user error, done from good intentions to "set NUM_CPU".

But the users should instead do:

`NUM_CPU 4`.

Both ways makes it possible to substitute in the value 4 with the substitution pattern ``, and if
all you use this for is to supply a value 4 to some forward model, both will work. But the DEFINE way of doing it will not lead to 4 processors being booked on the cluster

Also, if you have both

```
DEFINE 8
NUM_CPU 4
```

then 4 CPUs will be booked in the cluster, and the value `` will be replaced by 8, not 4, so the second definition of NUM_CPU is being shadowed with no warning given.

No file or test is named in the issue. Start by tracing ERT configuration parsing and the handling of DEFINE and NUM_CPU, then reproduce the two conflicting examples from the report. Done means DEFINE cannot silently shadow NUM_CPU, and the configuration clearly discourages or reports the incorrect form.

Written by the indexing model from the issue text.