Add conic projections to the projection gallery (#721)

* Add equidistant conic projection
* Add Lambert conic projection
* Add polyconic.py

Co-authored-by: Dongdong Tian <[email protected]>

Author: willschlitzer

examples/projections/conic/conic_equidistant.py

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+"""
+Equidistant conic
+=================
+
+``Dlon0/lat0/lat1/lat2/width``: Give projection center ``lon0/lat0``, two standard
+parallels ``lat1/lat2``, and the map width.
+"""
+import pygmt
+
+fig = pygmt.Figure()
+fig.coast(
+    shorelines="1/0.5p",
+    region=[-88, -70, 18, 24],
+    projection="D-79/21/19/23/12c",
+    land="lightgreen",
+    water="lightblue",
+    frame="afg",
+)
+
+fig.show()

examples/projections/conic/conic_lambert.py

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+"""
+Lambert Conic Conformal Projection
+==================================
+
+``Llon0/lat0/lat1/lat2/width``: Give projection center ``lon0/lat0``, two standard
+parallels ``lat1/lat2``, and the map width.
+"""
+import pygmt
+
+fig = pygmt.Figure()
+fig.coast(
+    shorelines="1/0.5p",
+    region=[-130, -70, 24, 52],
+    projection="L-100/35/33/45/12c",
+    land="gray",
+    borders=["1/thick,black", "2/thin,black"],
+    frame="afg",
+)
+
+fig.show()

examples/projections/conic/polyconic.py

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+"""
+Polyconic Projection
+====================
+
+``Poly/width``:  The only additional argument for the projection is the map width.
+"""
+import pygmt
+
+fig = pygmt.Figure()
+fig.coast(
+    shorelines="1/0.5p",
+    region=[-180, -20, 0, 90],
+    projection="Poly/12c",
+    land="gray",
+    borders="1/thick,black",
+    frame="afg10",
+)
+
+fig.show()