Random uncorrelated offsets

Add uncorrelated random offsets in instrument.py and config file

Add keyword instrument.offset.sigma1d in desi.yaml configuration file. Default value of 5.1 um is 1/sqrt(2) of quadratic sum of all contributions to lateral shift in DESI-347-v11 excluding the 'Band-limited chromatic shift' (already included in radial centroid offsets) and 'FVC corrector optics figure error' (because this is mostly static)
Rename instrument.offset.random keyword to instrument.offset.static because it emulates the effect of the corrector lens polishing errors (see DESI-1071), which is mostly static/stable.
Comment out instrument.offset.static in default configuration because we should hope that a large fraction of this effect will be calibrated and corrected during commissioning. We will change this when a more precise model of lateral shifts becomes available (for the moment, some contributions are missing: atmospheric jitter, telescope guiding/tracking errors, knowledge of static telescope optical distortion, truss and telescope lateral thermal drift, and the others are not very well known).

desihub / specsim