Support XML string as well as file path in NDAttributesFile

[coretl]

In bluesky we would like to configure an areaDetector without having to have a shared filesystem with the detector. This is almost possible at the moment, but if we need to set NDAttributesFile then we need to pass a filesystem path which is read here:

ADCore/ADApp/ADSrc/asynNDArrayDriver.cpp

Lines 329 to 338 in 9d2c32a

	infile.open(fileName.c_str());
	if (infile.fail()) {
	asynPrint(pasynUserSelf, ASYN_TRACE_ERROR,
	"%s::%s error opening file %s\n",
	driverName, functionName, fileName.c_str());
	setIntegerParam(NDAttributesStatus, NDAttributesFileNotFound);
	return asynError;
	}
	buff << infile.rdbuf();
	buffer = buff.str();

The pos plugin has a similar XML reading routine, but it supports the PV being either a blob of XML or a filename:

ADCore/ADApp/pluginSrc/NDPosPluginFileReader.cpp

Lines 65 to 69 in 9d2c32a

	if (filename.find("<pos_layout>") != std::string::npos){
	xmlreader = xmlReaderForMemory(filename.c_str(), (int)filename.length(), NULL, NULL, 0);
	} else {
	xmlreader = xmlReaderForFile(filename.c_str(), NULL, 0);
	}

I propose we do something similar for NDAttributesFile:

if (fileName.find("<Attributes>") != std::string::npos){ 
    buffer = fileName;
} else {
    // The existing code
} 

Is this a reasonable thing to do? If so, then we can make a PR for it.

[MarkRivers]

That seems reasonable to me.

[coretl]

@marcelldls please could you take a look at this?

[MarkRivers]

Closed via #503