Allow `.DISP` to be changed from the ioc side

[evvaaaa]

In pandablocks-ioc we'd like to set the DISP of some PVs so that they can't be changed during HDF5:Capture. The only way to set this with the current pandablocks-ioc is through caput. It'd be nice to be able to set DISP from the ioc side.

[AlexanderWells-diamond]

Summing up a chat I had with @Araneidae:

• This will require a new function in ProcessDeviceSupportCore, probably called setField. Parameters will be the field name and new value
• It'll need to do some processing like what is currently done in this set method.
• The value being written will need to be coerced into a DBR_STRING type - all fields are either numbers or strings, which should both be coercible from string without an issue

[Araneidae]

This should be relatively straightforward to implement. I'm no longer set up to actually run pythonIoc anymore, so try the following untested patch:

diff --git a/softioc/device.py b/softioc/device.py
index 4fcb5ad..9f5820b 100644
--- a/softioc/device.py
+++ b/softioc/device.py
@@ -83,6 +83,12 @@ class ProcessDeviceSupportCore(DeviceSupportCore, RecordLookup):
     def _write_value(self, record, value):
         record.write_val(value)
 
+    def set_field(self, field, value):
+        data = (c_char * 40)()
+        data.value = str(value).encode() + b'\0'
+        db_put_field(
+            self._record.NAME + '.' + field, fields.DBF_STRING, data, 1)
+
 
 class ProcessDeviceSupportIn(ProcessDeviceSupportCore):
     _link_ = 'INP'