BTN-2427 Add tied array beams

Pulsar positions (for timing specifically, an AA0.5 activity) can go as a new object in the targets sections, as the same level as radial velocity or pointing_pattern.

This should be done using ADR-63 syntax i.e. 'c1' and 'c2'. Note that the ADR-63 schema allows for 'parallax' as part of the icrs definition and so we should allow it (we actually want to use the same target schema everywhere) but it definitely needs to be optional.

"beam_coordinate" is analogous to "reference coordinate" which is used for the target to which the tied-array beam targets are attached.

The station weights ("stn_weights") are a mandatory parameter in the OSO-TMC schema. They should be used in a binary fashion i.e. each station can be either 0.0 or 1.0 depending on whether that station should actually be used when forming the tied-array beam. OSO should remain flexible and continue to allow float values.