Add logging station location directives

[rsuzuki0]

Summary

• add myaltitude, mycity, mystate, mycountry, and mydxcc header directives
• propagate the station location values to every parsed QSO
• export them as MY_ALTITUDE, MY_CITY, MY_STATE, MY_COUNTRY, and MY_DXCC
• validate finite numeric altitude and non-negative DXCC code syntax
• normalize and validate U.S. mycounty values as STATE,County when the logging-station DXCC is 6, 110, or 291
• document the directives and add unit plus end-to-end tests

Relationship to #172

This is the input-syntax expansion step and is intentionally separate from #172. PR #172 corrects and enriches output for location and program-reference inputs FLEcli already supports. Both changes are complementary; #172 should ideally merge first so existing decimal mylat/mylon values are also encoded correctly.

ADIF version

MY_ALTITUDE requires a newer ADIF specification than the currently declared 3.1.0, so this PR updates ADIF_VER to 3.1.7 independently.

Validation

• go test ./...
• go test -race -count=1 ./...
• go vet ./...