Add Simrad `.idx` and `.bot` content via `open_raw`

[leewujung]

So far in converting Simrad data we only parse the .raw files. Since .idx and .bot files also contain useful information and the format is simple, let's also parse them and include the data into the EchoData object.

EK80 reference:

• .idx file: https://www.simrad.online/ek80/interface/ek80_interface_en_us.pdf#page=174
• .bot file: https://www.simrad.online/ek80/interface/ek80_interface_en_us.pdf#page=178

Tasks

• [ ] add mechanisms to associate and parse .idx and .bot files in open_raw
  • datagram parser already exists as part of ek_raw_parsers.py from PyEcholab
• [ ] store parsed data from these files into EchoData object
  • content of .bot can probably go to Platform or Vendor_specific group?
  • content of .idx:
    • lat/lon overlaps with what's parsed from .raw file -- should we add postfix to variable names like latitude_idx?
    • vessel distance can be stored in Platform group