create Electrodes class to replace electrodes table

[rly]

@ajtritt I am trying to create an Electrodes class to use for the electrodes table instead of a DynamicTable to solve #1204. Without being its own class, we cannot specify in PyNWB that certain columns of the electrodes table are optional.

This involves changing the ObjectMapper for NWBFile such that it constructs an Electrodes object instead of a DynamicTable object for the NWBFile.electrodes constructor argument.

I thought of a few ways to do it:

1. Construct an Electrodes object from the /general/extracellular_ephys/electrodes/ group builder by setting each value and creating each VectorData individually, but this would be very tedious and involve a lot of hard coding.
2. Construct an Electrodes object from the /general/extracellular_ephys/electrodes/ group builder by setting the values of the Electrodes object to the values of the DynamicTable that is already generated. This doesn't work because I cannot change the parent of the VectorData classes to the new object. That is what I have coded up in the PR here.
3. Override the container class associated with the /general/extracellular_ephys/electrodes/ group builder. This doesn't seem possible.

Can you think of a better way to do this? Would this even be possible? Would it require substantial changes to HDMF to work?

Stepping back at the source problem, it might be easier to change how adding columns works in DynamicTable to handle optional columns. What do you think?

[rly]

I thought of another (slightly hacky) way to get around this issue. We can just promote the DynamicTable object constructed by the ObjectMapper to be an Electrodes object and add the new columns to the class. I updated the PR to reflect this method. This requires hdmf-dev/hdmf#323

[rly]

@mavaylon1 is restarting this effort with a new ElectrodesTable neurodata type in #1890. Will close this PR.