Morecantile

* add `to_proj4` and `to_dict` and `to_json` methods to `CRSType`
* remove `TileMatrixSet._crs` (replaced with `TileMatrixSet.crs._pyproj_crs`)

* add `to_epsg()` and `to_wkt()` methods to `CRSType` to allow compatibility with `4.0`

python
import morecantile

tms = morecantile.tms.get("WebMercatorQuad")

tms.crs
>> CRSType(__root__='http://www.opengis.net/def/crs/EPSG/0/3857')

tms.crs.to_epsg()
>> 3857

tms._crs
>> <Projected CRS: EPSG:3857>

tms._crs.to_epsg()
>> 3857

* Change CRS attribute in model to align more with the TMS spec and fix some OpenAPI schema issues (It should be a string URI or WKT, not a pyproj.CRS)
* add `TileMatrixSet._crs` PrivateAttr to host the `pyproj.CRS` version of the crs
* update `grid_crs` properties in `TileMatrixSet.feature()` result to return the `TileMatrixSet.CRS` (uri/wkt) instead of EPSG code

* Fix TileMatrixSet BoundingBox definition (https://github.com/developmentseed/morecantile/pull/122)

* Raise a `ValueError` when `nan` bounds are passed to `tiles` (author samn, https://github.com/developmentseed/morecantile/pull/120)

* no change since `4.0.0a1`