Export feature "age" to MVT.HEADmaster

This allows client-side filtering.

The "age" attribute is a signed short (int16) expressing the number of
days since 1970-01-01.  This covers the range 1880-04-15 to 2059-09-18
which should be more than enough.  The source value is a Date or
Datetime and depends on the source layer.

  - For vbk:*, it's the date at which the project was last saved in
    Vindbrukskollen.
  - For mrr:*, it's the date at which the application came to
    Bergsstaten for applications, and decision date for granted permits.
  - For avverk:*, it's the date at which the application came to
    Skogsstyrelsen for applications, and the clearcut date (according to
    Skogsstyrelsen) for completed objects.

For other layers, we don't export that attribute.

This makes the tiles a bit larger.  Before (without the extra
attribute):

  vbk: 1261× tiles, 599kiB uncompressed (avg=487B/t), 389kiB brotli (35%, avg=316B/t)
  avverk: 3734× tiles 115MiB uncompressed (avg=32kiB/t), 72MiB brotli (37%, avg=20kiB/t)
  mrr: 1324× 331kiB uncompressed (avg=257kiB/t), 289kiB brotli (13%, avg=223B/t)
  → total 121631367B uncompressed (avg=19kiB/t), 76692807B brotli (37%, avg=12kiB/t)

After (with the extra attribute):

  vbk: 1261× tiles, 714kiB uncompressed (avg=580B/t), 425kiB brotli (40%, avg=345B/t)
  avverk: 3734× tiles 127MiB uncompressed (avg=35kiB/t), 78MiB brotli (39%, avg=21kiB/t)
  mrr: 1324× 323kiB uncompressed (avg=322kiB/t), 342kiB brotli (18%, avg=265B/t)
  → total 134274796B uncompressed (avg=21kiB/t), 82264731B brotli (39%, avg=13kiB/t)

Summary:

  +12.1MiB uncompressed (+10.4%, avg=+1.95kiB/t)
  +5.3MiB compressed (+7.3%, avg=+882B/t)

0 files changed, 0 insertions, 0 deletions