Support arrays whose length is determined by another field

[ddasilva]

This feature request comes from wanting to support a type of data I have been told some instruments generate, but currently isn't supported.

This change would update VariableLength class to support variable length fields where the length is determined by another field in the packet. For example, you could have fields data1_len which sets the length of the data array, and then data2_len which follows it and sets the length of the data2 array.

An example is below:

   import ccsdspy
   from ccsdspy import PacketField, PacketArray

    pkt = ccsdspy.VariableLength([
         PacketField(
              name='SHCOARSE',
              data_type='uint',
              bit_length=32
         ),
         PacketField(
              name='data_len',
              data_type='uint',
              bit_length=8,
         ),  
         PacketArray(
              name="data",
              data_type="uint",
              bit_length=16,
              array_shape="data_len",  # links data to data_len
         ),
         PacketField(
              name="checksum",
              data_type="uint",
              bit_length=16
         ),
    ])