Design of absolute photometry AOTs

The considerations given in "Introduction and purpose of PHT05 and PHT25"
have led to the following design of the absolute photometry AOTs:

* All measurements are in staring mode to allow the best signal stabilization.

* Only one filter and one aperture (in case of PHT-P) is selected per
  observation.

* A dark signal measurement obtained by blocking the optical path to the
  detector can be included. The integration time is set to 256 sec.

* A measurement of the FCS with no heating power applied (cold FCS) can be
  included. This measurement provides a correction for the straylight
  component. The integration time is set to 256 sec for the P1, P2, P3, and
  C100 detectors. For C200 the time is set to 128 sec.

* The measurement against the heated FCS is always included. The FCS flux
  level is set to the same value as the expected sky flux level. The FCS
  measurement has the same integration time as the sky measurement.

The measurement sequence is as follows:

- (optional) Dark	256 sec
- (optional) Cold FCS	256 sec for P1, P2, P3, and C100
			128 sec	for C200
- Sky			N sec
- FCS			N Sec

where N is an integer power of 2 with a minimum of 32 sec. Recommendations
on the Sky (and consequently FCS) times are given on page
"What is the optimum integration time".

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