Support values of union DataTypes.

[MurzynJ]

• Importing a value works, if its DataType has been loaded through the NodesetCompiler before. (Trust me I tested it)
• Importing of union DataTypes from XML seems to still be broken.

Due to the above it's hard to make a unit test at the moment, as I can't get a DataType into the loader in the first place, without creating a dependency on open62541 code (to get the NodesetCompiler).

It would be useful to have a look in future at importing of union DataTypes.

[codecov-commenter]

Codecov Report

Merging #184 (b7943d1) into master (6e1da3d) will decrease coverage by 0.51%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
- Coverage   93.42%   92.90%   -0.52%     
==========================================
  Files          57       57              
  Lines        3969     3991      +22     
==========================================
  Hits         3708     3708              
- Misses        261      283      +22     

Impacted Files	Coverage Δ
backends/open62541/src/Value.c	81.12% <0.00%> (-10.26%)	⬇️

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[MurzynJ]

@matkonnerth
Have you had a look at this change? I'm hesitating to merge it yet, because I didn't come up with a good test, that would make a pass through the added code (that's why the coverage % dropped). As I wrote in the fist comment it would be easier to make the test if importing of the Union DataTypes works correctly (see #186).

[matkonnerth]

Hi @MurzynJ , thanks for your work, importing union datatypes should be fixed now.