Corner cases in ft_redefinetrial (trials selection + variable offset/toilim)

[NirOfir]

Describe the bug
ft_redefinetrial can fail (either crash or choose the wrong data) if the user tries to both make a selection of trials and make a trial-by-trial offset.

To Reproduce

% Create dummy data
data = [];
data.trial = mat2cell(randn(3, 4004), 3, [1001 1001 1001 1001]);
data.time  = repmat({linspace(0, 1, 1001)}, 1, 4);
data.label = {'A', 'B', 'C'};

% This incorrectly shifts the second trial by 0.3 seconds
cfg        = [];
cfg.trials = [false, true, false, false];
cfg.offset = [-500, -300];
data_out = ft_redefinetrial(cfg, data);
data_out.time{1}(1)

% This throws an error because it looks for more offsets than it got
cfg        = [];
cfg.trials = [false, true, true, false];
cfg.offset = [-500, -300];
data_out = ft_redefinetrial(cfg, data);

% This throws an error because not enough toilims are provided
cfg        = [];
cfg.trials = [false, true, true, true];
cfg.toilim = [0, 0.3; 0.3, 0.5];
data_out = ft_redefinetrial(cfg, data);

The 3rd bug happens because the code uses length(cfg.trials) even when it is logical and sum(cfg.trials) should be used instead in the following lines:

fieldtrip/ft_redefinetrial.m

Lines 142 to 150 in 3be5222

	if length(cfg.offset)>1 && length(cfg.offset)~=length(cfg.trials)
	cfg.offset = cfg.offset(cfg.trials);
	end
	if length(cfg.begsample)>1 && length(cfg.begsample)~=length(cfg.trials)
	cfg.begsample = cfg.begsample(cfg.trials);
	end
	if length(cfg.endsample)>1 && length(cfg.endsample)~=length(cfg.trials)
	cfg.endsample = cfg.endsample(cfg.trials);
	end

Although it's possible to patch the code and keep the current (silent) "correction" of offsets and add a correction for toilims too, I think that the code should be stricter with users instead of guessing what they want, and return a clear error if the number of chosen trials is inconsistent with the number of offsets/toilims. If you think that makes sense I'll be happy to make the PR.

[NirOfir]

I made a PR which I think corrects for the original intention (select from offset/toilims if the user provided those for all of the original data along with cfg.trials). I added a check that the number of offset/toilims is consistent, and expanded the help a bit.
The PR returns hopefully informative error messages for cases 1 & 3 in the test code and works as expected for case 2.