Helper for concatenating disparate table rows

[shippy]

Matlab cannot concatenate tables with different number of columns/differently named columns, but there are times where this could be advantageous without paying overt risk - e.g. random appearance of phase, changing of internal structure between two sessions, etc. Basically, a column-wise version of intersect.

There are at least two critical points where this will cause the framework to have a hard fail: runBlock>appendRow and exportParticipant/exportTaskData.

The rough sketch of the helper:

1. uniqueToA = setdiff(A.Properties.VariableNames, B.Properties.VariableNames). Iterate over uniqueToA and add all of these with addConstantColumnToTable.
2. Same principle: uniqueToB = setdiff(B.Properties.VariableNames, A.Properties.VariableNames).
3. Concatenation should now work. If concatenation errors out, it's because two columns have different types. This is desirable in the addBlock case; less so in the export case.

The main pitfall is generating the right empty values for the type. The low-level version of this is a lookup table; the more elegant version is varTypeFn = str2func(class(A.(column)); B.(column) = varTypeFn.whateverFunctionMakesTheDefaultValue; (.empty is great, but won't work, since it's the MATLAB idiom for deletion); I should look into standardizemissing and fillmissing, if only for inspiration of missing values by type.

[shippy]

Here's a MathWorks forum answer, which has been noted #12.