add warning when non wgs84 geographic crs and optional wgs84 default

[vincentsarago]

closes #165

This PR does:

• add _geographic_crs private attribute (again)
• add 2 env variable control:
  • MORECANTILE_IGNORE_NON_WGS84_GEOGRAPHIC: silent warning when geographic CRS is not WGS84
  • MORECANTILE_WGS84_GEOGRAPHIC: make WGS84 the default geographic CRS

@AndrewAnnex any opinions?

[AndrewAnnex]

@vincentsarago for this PR since the TMS spec doesn't require WGS84, I feel like warnings related to WGS84 not being used should only be isolated to areas that actually may require it like GeoJSON in #165 (although I will comment there also, I don't like how RFC7946 is used in practice.. but more on that later in #165 comments). Looking at all the new warnings you need to configure to ignore in the pyproject.toml seems to confirm that to me.

Also this is just a bit of "explicit is better than implicit", like if someone uses NZTM2000Quad they know what they are asking for.

That said warnings are 10e10 more preferable to throwing exceptions. I just think it might be better to have the environment variables be a "opt-in" rather than an "opt-out".

For 99% of users, not issuing warnings when they wouldn't occur anyways is fine, for the 1% that know they aren't using WGS84 in their requests a warning about it doesn't help them. But if someone wants to be warned then I think they can enable to config.