Now description present in sensor_list is super type one.
So we revert to sensor super type management

This change make easiest comparasaison between gresp and wdataless
production

Suppress super type management

Add new test to verify that transfer normalization frequency is defined
for analog PZ filter

  - Set sensitivity and sensitivity frequency (blockette 58) for IIR_PZ (Sensitivity)
    and IIR_COEF (Frequency of gain)
  - Add management of OFFSET keyword for blockette 57
  - Unify function (process_digital_filter) for digital_filter and digital_filter_stage

Copy comment list reference when split station by network.

Fix constant renaming from IRR_POLY to IRR_COEF
Fix some potential bug in sprintf call

The station begin and end time are no more set by station split
procedure, so we fix it according to dbirs information.

We use only network channel to duplicate station.
Some code cleaning

We fix ending time of station, channel and comment to
23:59:59.9999 31/12/2500 when is null.

Update to conform with august 2012 SEED version.
Now when a stage of channel is a polynomial one, the stage 0 must be
of type polynomial instead of not present.

Now we apply new convention
  Z -> Z
  N -> 1 instead of 2
  E -> 2 instead of 3
Some bug fix and improvements