[New Check]: Add check electricalseries of dtype int16

[alessandratrapani]

Fix #395

Add check on ElectricalSeries data type: if the conversion and offset are DEFAULT values and the data type is any integer ("uint16", "int16", etc).

• add check function
• add unit tests
• add check function documentation in ecephys.rst
• update CHANGELOG.md

[CodyCBakerPhD]

Just occurred to me while inspecting some files on the GUIDE; we should also consider the channel_conversion array in this check

If the attribute exists, and the elements of it are not also 1.0, then it's fine to have conversion=1.0 and offset=0.0 because the array of conversion factors is the intended way of specifying that information