Merging datasets is not properly handled

[juanangp]

I found an issue merging datasets using TRestDataSet::Import function implemented here: https://github.com/rest-for-physics/framework/blob/9c2fae713e932c11c23efc78e332bdac2e3dad5e/source/framework/core/src/TRestDataSet.cxx#L952

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