Lack of support for special letters like ø æ å

**Describe the bug**
_roffio.parser.as_ascii decodes string fields with bytes.decode("ascii"), which raises UnicodeDecodeError when a ROFF file contains any byte ≥ 0x80 in a string (e.g. parameter codeNames). This makes the whole file unreadable even though only a single string entry is affected, and even though all numerical data is intact.

**To Reproduce**
Seen through xtgeo

```
import xtgeo
g = xtgeo.create_box_grid(dimension=(4,2,4))
p = xtgeo.GridProperty(g, discrete=True)
p.codes = {0: "øæå"}
p.to_file("some.roff")
```

triggers

`UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in range(128)`

**Expected behavior**
Should be able to read and write code names with e.g. norwegian special letters

**Desktop (please complete the following information):**
- OS: Linux (etc, I suppose)
- Version RHEL8

**Additional context**
RMS can write ROFF files with e.g. code names "Havsø", but reading that with roffio (in xtgeo) will fail

Start at roffio.parser.as_ascii and reproduce the failure with the xtgeo example using code names such as "øæå". Trace the string decoding and writing paths, then verify that ROFF files with non-ASCII code names can be read and written without making numerical data unreadable.

Written by the indexing model from the issue text.