Add $ wrapper to @ for ATO objects

[mhpob]

This PR adds a method to the $ generic for ATO objects. Nothing fancy like the get_ functions -- just a wrapper around slot(object, name) with a warning that this was done for convenience and @ should be used in the future.

Scroll to line 184 in R/get_methods.R to review -- everything before that was automatic formatting from Air.

r$> ato <- init_ato()

r$> ato$det
Warning in ato$det :
  `ato$det` converted to `ato@det` for convenience, please use `ato@det` in the future.
  See `?slot` for more information.
[1] datetime        frac_second     receiver_serial transmitter     sensor_value    valid
<0 rows> (or 0-length row.names)

r$> ato$det$datetime
Warning in ato$det :
  `ato$det` converted to `ato@det` for convenience, please use `ato@det` in the future.
  See `?slot` for more information.
POSIXct of length 0

r$> ato$dep
Warning in ato$dep :
  `ato$dep` converted to `ato@dep` for convenience, please use `ato@dep` in the future.
  See `?slot` for more information.
 [1] receiver_model           receiver_serial          receiver_codeset         deploy_location          deploy_datetime
 [6] deploy_lat               deploy_lon               deploy_z                 recover_datetime         recover_lat
[11] recover_lon              transmitter              transmitter_manufacturer transmitter_ping_rate    transmitter_model       
[16] transmitter_serial      
<0 rows> (or 0-length row.names)