Add additional TAP_SCHEMA data to the Felis file format

The TAP_SCHEMA data model for the schema of a set of data exposed via TAP includes a variety of elements that are intended to provide hints to user interfaces for how the organization of the data should be presented in interfaces. These include:

For "schemas"

In TAP_SCHEMA.schemas, the description and schema_index attributes for a "schema" (in the TAP sense) affect the UI presentation. schema_index, in particular, defines a suggested order of presentation of the "schemas" in a user interface.

For tables

In TAP_SCHEMA.tables, the description and table_index attributes for a table. From the standard:

Clients may order by table_index (ascending) so lower index tables would appear earlier in a listing.

For columns

In TAP_SCHEMA.columns, the description, principal, and column_index attributes for a table. From the standard:

The principal, indexed, and std columns are boolean values implemented as integers. As such, the value must be 0 or 1; no other values are allowed.

The principal flag indicates that the column is considered a core part of the content; clients can use this hint to make the principal column(s) visible, for example by selecting them by default in generating an ADQL query. In cases where the service selects the columns to return (such as a query language without an explicit output selection), the principal column indicates those columns that are returned by default.

(...)

The column_index is used to recommend column ordering for clients. Clients may order by column_index (ascending) so lower index columns would appear earlier in a listing. This is useful for keeping related columns together in output or display.

Note that there is a single TAP_SCHEMA.columns table for all the tables on a TAP service. So all the column_index values appear in a single table. However, they are primarily only meaningful in the context of the display of attributes of a single table, so it is OK if columns from different tables share a column_index value.

In general

For the *_index attributes, as they have to define an ordering, it may not be appropriate for them to be sourced "locally" with the definitions of the tables and columns in the Felis source files. Most likely the system that takes Felis definitions and produces TAP_SCHEMA content from them needs an additional input source that can provide the ordering "after the fact".

(The following is not part of the task of this ticket, it's here just for context.) We should establish a pattern for the description elements - an approximate length range for the content, for instance, and a standard for their character representation (UTF-8?) and markup, if any.