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fixing annoying bug: only the first cavity with 3D fieldmap got correct results; reported by N. Neveu

tell H5hut not to abort on error, we check the return value; fixing OPAL/regression-tests#11 OPAL/regression-tests#12 OPAL/regression-tests#13

First proposition for better error messages

With this patch the user gets a hint about what is wrong in his input file as requested in #32 

If one omits the TRACK command OPAL writes
```
Error>     RUN,METHOD="PARALLEL-T",BEAM=BEAM1,FIELDSOLVER=FS1,DISTRIBUTION=DIST1;
Error>     Syntax error, the command 'RUN' could belong to
Error>       - the command TRACK
Error>     but it's not present!
```

If parts of a definition are commented out then OPAL writes
```
Error>     MAXSTEPS=600000,DT=2e-13,ZSTOP=0.3;
Error>             ^
Error>     Syntax error, either the keyword REAL is missing or
Error>     the attribute 'MAXSTEPS' could belong to
Error>       - the command TRACK
Error>     but it's not present!
```

Disadvantage: one has always to use REAL when defining variables. e.g.
```
MINSTEPFORREBIN = 500;
```
isn't accepted any more. Instead one has to write
```
REAL MINSTEPFORREBIN = 500;
```

See merge request !9