add current

TODO

Add properties to access the electric_current like:

from tomoscan.esrf.scan.hdf5scan import HDF5TomoScan
scan = HDF5TomoScan("my_file.nx", "entry0000")

frames_electric_current = scan.electric_current

This will return the array of all the electric_current. Dark and flat frames as the projection or the alignment one. To access some frames users can filter those values from the image_key_control values.

Like:

from tomoscan.esrf.hdf5scan import ImageKey

projections_electric_current = scan.electric_current[scan.image_key_control == ImageKey.PROJECTION.value]