Allow setting default params when creating device reader

[glopesdev]

This feature allows setting a default reference epoch and other common params for all register readers when creating the device reader from a schema or folder, to make it easier to setup consistent data frame outputs and reduce bookkeeping.

For example, you can set a common reference epoch at the top and then get consistent time indices for all dataframes:

from harp.io import REFERENCE_EPOCH

reader = create_reader('device.ylm', epoch=REFERENCE_EPOCH)
reader.WhoAmI.read()

It is also possible to override the parameters for individual reads, e.g. reader.WhoAmI.read(keep_type=True) will make the type column visible no matter what the default setting is.