Proposal to simplify NWBFile and implementation of new neurodata_types

[oruebel]

Currently NWBFile defines custom create methods for various different neurodata_types. To simplify NWBFile and centralize logic specific for a particular type within the type, I propose to:

• Move the NWBFile::createElectricalSeries methods to the corresponding container class (here ElectricalSeries) and implement them as static factory method that generate the type and add it to the NWBFile. I.e., the method would then need to take the NWBFile as input (which would also already include the I/O object). We could in principle just have these as variants of the create methods that we already have by overloading it or use another name, e.g, createInFile