Nasa DEM

## Did you read the Contributor Guide?

- Yes, I have read [How to contribute](https://github.com/wri/cities-cif#How-to-contribute)

- No, I haven't read it.

## Is this PR related to a JIRA ticket?

- Yes, the URL of the associated JIRA ticket is https://gfw.atlassian.net/browse/CIF-XXX. The PR name follows the format `[CIF-XXX] my subject`.

- No, this is a documentation update. The PR name follows the format `[DOCS] my subject`.

## What changes were proposed in this PR?

## How was this patch tested?

## Did this PR include necessary documentation updates?

- Yes, I am adding a new API feature.
- Yes, I have updated the documentation update.
- No, this PR does not affect any public API so no need to change the docs.

city_metrix/layers/__init__.py

@@ -17,3 +17,4 @@
 from .tree_canopy_height import TreeCanopyHeight
 from .alos_dsm import AlosDSM
 from .overture_buildings import OvertureBuildings
+from .nasa_dem import NasaDEM

city_metrix/layers/nasa_dem.py

@@ -0,0 +1,19 @@
+import ee
+import xee
+import xarray as xr
+from city_metrix.layers.layer import Layer, get_image_collection
+
+class NasaDEM(Layer):
+    def __init__(self, **kwargs):
+        super().__init__(**kwargs)
+
+    def get_data(self, bbox):
+        dataset = ee.Image("NASA/NASADEM_HGT/001")
+        nasa_dem = ee.ImageCollection(ee.ImageCollection(dataset)
+                                      .filterBounds(ee.Geometry.BBox(*bbox))
+                                      .select('elevation')
+                                      .mean()
+                                      )
+        data = get_image_collection(nasa_dem, bbox, 30, "NASA DEM").elevation
+
+        return data