Merging datasets is not properly handled

[juanangp]

I found an issue merging datasets using TRestDataSet::Import function implemented here:

framework/source/framework/core/src/TRestDataSet.cxx

Line 952 in 9c2fae7

void TRestDataSet::Import(std::vector<std::string> fileNames) {

Although the trees are properly merged the metadata has issues, for instance while merging two datasets the duration is not properly retreived.

I guess we should implement a TRestDataSet merging of the metadata to solve this issue