Create a `headers` key in the digitizer mapping `configs` dict

[rocco8773]

For digitizer mapping configs dict, create a 'headers' key. This would contain mapping details for the header dataset, such as, the 'shotnum' key for specifying the shot number column, a 'offset' key for specifying the voltage offset, etc.

Would also need to update code-blocks in HDFReadData related to defining shotnumkey and conditioning keep_bits.