Change OBS_ID keyword from integer to string

[jknodlseder]

The event list extension has an OBS_ID keyword that is currently defined as integer. This comes from the original event list format specification from Karl. It appears however that OGIP has in the meantime switched to a string, see for example https://heasarc.gsfc.nasa.gov/docs/heasarc/ofwg/docs/general/ogip_94_001/ogip_94_001.html which says:

OBSERVATION ID: Sequence numbers are commonly used by missions to identify 'uniquely' a particular dataset. The definition of the sequence number is not unique across mission and typically depend upon the different way a dataset is identified within that mission. Although sequence number suggests a numerical value only, past mission had given the sequence number as a mixture of numerical and character values. To avoid a proliferation of keywords to store the sequence number value, the OGIP recommend to use OBS_ID. The keywords value is a string, to allow back compatability with the already archived mission. The OBS_ID keyword value can be used by database software.

So I would propose to change the OBS_ID keyword from integer to string.