local calibrations: names can be too long + definition of macros

Currently the danfysik and eurotherm create a local calibration directory as:

```
epicsEnvSet "CALIB_BASE_DIR" "$(ICPCONFIGBASE)/$(INSTRUMENT)"
```

This has multiple issues:
- If `$(INSTRUMENT)` is too long, CALIB_BASE_DIR can no longer be passed to the `cvt` record (it causes a DB load failure)
* We could solve this by increasing the allowed size in the `cvt` record.
- `$(INSTRUMENT)` does not include the `NDX/NDE/NDA` prefix, meaning that both `c:\instrument\settings\config\NDXMERLIN` and `c:\instrument\settings\config\MERLIN` are needed on an instrument. The second of these is *not* version controlled and should be this is not good.
* We don't see this on developer machines as on dev machines `$(INSTRUMENT)` == `$(COMPUTERNAME)`, but this is not true on instruments

We may need some wider discussion (and documentation) on exactly what each of these variables are and how they are meant to be used.

## Acceptance Criteria
* We have a standard way of using local calibration directories
* This is enforced on all instruments and doesn't break if paths are too long
* There are tests for this