[BUG] Warning and error messages in matRad_readNRRD fail, string concatenation is wrong when lineContent is an empty cell

[zperko]

Describe the bug
When reading NRRD files and seeing empty lines or other content for which a warning/error is raised, the warning/error messages themselves are wrong, the string concatenation doesn't work with lineContent being an empty cell.

To Reproduce
Read and NRRD file that would raise a warning/error according to the lineConcent in matRad_readNRRD.m

Expected behavior
Error/warning line just executes properly, raising the error/warning.

Solution
The strings in the warning/error messages should be properly formatted, an exception for empty cell should work. Might be an issue in several other functions too.

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. Windows]
• Environment & Version [e.g. Matlab 2020b]
• matRad Version [e.g. 2.10.0] If developing with git, state the branch and commit ID, if possible

Additional context
Add any other context about the problem here.

[zperko]

For lines 63 and 71 lineContent should probably be currentLine

[wahln]

Yeah, this block:

matRad/IO/matRad_readNRRD.m

Lines 61 to 71 in cb6b04a

	lineContent = regexp(currentLine, '(.+):(=|\s)(.+)', 'tokens');
	if isempty(lineContent)
	warning(['Could not parse line: "' lineContent '"']);
	elseif isequal(lineContent{1}{2},' ') %space after colon refers to "field"
	nrrdMetaData.fields{end+1,1} = lineContent{1}{1}; %Fieldname
	nrrdMetaData.fields{end,2} = lineContent{1}{3}; %Information
	elseif isequal(lineContent{1}{2},'=') %= after colon refers to key-value pair
	nrrdMetaData.keys{end+1,1} = lineContent{1}{1}; %Key
	nrrdMetaData.keys{end,2} = lineContent{1}{3}; %Value
	else
	warning(['Could not parse line: "' lineContent '"']);

We should replace the formatting like we do in other functions using the printf-like syntax and, as you said, in line 63 and 71, use currentLine.