Document almanac, update for new libswiftnav / piksi_firmware definition

[denniszollo]

Documented, generalized almanac message using gps_time_t type for time and full week number.

requires changes here: https://github.com/swift-nav/libsbp/blob/master/spec/yaml/swiftnav/sbp/piksi.yaml#L25

linked to https://github.com/swift-nav/libswiftnav/issues/217 and swift-nav/piksi_firmware#550