Improve SINEX covariance information

[ccrook]

See https://github.com/linz/national-geodetic-adjustment/issues/9

Want the ability to add centring uncertainty to SINEX data - independent E/N/U error (or maybe just H/V error) at each station. Ideally will include identifying CORS stations and not applying to them. For convenience in NGA might want to be able to apply globally (OTOH may be better to explicitly include in data_file command for transparency).

Maybe something like...

data_file .... sinex centring_error h mm v mm [for/except list ... end_list]