[PROPOSAL] Create a chimera weather station instruments and script

[wschoenell]

This proposal is to finish the weather station instrument interface, incorporating it to provide metadata to the camera and the possibility to change operations according to the weather. The actual implementation can be seen at b4e2b31.

The changes mapped by are listed below. Feel free to add/comment on them.

• - Remove deltaT from the interface. I think that what makes more sense is to maybe return the measurement with the time since it was done, e.g.::

def temperature(unit=degC):
    timestamp, temperature = self._query_temp()
    return temperature, time.time() - timestamp

This avoids the need to one implement some kind of complex database to store data making it available back in time. Also, if one checks the weather station say every 10 minutes, the date of the meteorological observation is retained.

• - Improve the documentation of the interface with comments.
• - Create a FakeWeatherStation instrument and test it. The measurements of the weather station should be incorporated to the global metadata, appearing on a exposure. Fits keywords could be the ones that are actually on the interface plus a keyword with the date/time of the weather measurement (i.e. METDATE, with the same standard of DATE-OBS)

Items 1 to 3 must be released on one PR

• - Create a chimera-weather script. Working only for --info purposes or create it as a controller which decides, for example, to close the observatory? I think that the second option could be more elaborate and separated from the weather, taking in account on more variables.

Item 4 must be released on another PR

[wschoenell]

Items 1 to 3 are being discussed on PR #96

[wschoenell]

1 to 3 fixed on #96.