add: summary level database descriptor
add: summary level project descriptor
add: version level descriptor
add: distribution level component descriptor
add: distribution level aggregate descriptor
rm: generic descriptor
add: experiment project level
add: experiment study question level
add: experiment mr study level
add: experiment acquisition level
Merge pull request #153 from nicholsn/enh/dataset-descriptors

Updated examples of dataset descriptors
Merge pull request #154 from nicholsn/enh/experiment

Examples of NIDM Experiment