default dataset path

[mthielma]

There is an issue when running GMG_Tomo() from an arbitrary folder. In that case, the search path is relative to the current directory. For this reason, Default_datasets.jl and GMG_TomoData.jl were not found. The recent fix consists of using the output of pkgdir(GMG_DataPicker) as a prefix. However, when GMG_Tomo is then called with a specific path+filename, things don't work anymore.
I'd suggest to replace the argument default_datasets="src/GMG_Tomo/Default_datasets.jl" with datasets=nothing and add an if-loop that sets the path to the default path in case datasets equals nothing