Add Process CSV button

This is to prevent re-processing the uploading CSV file when
a user clicks the download button.

app.py

@@ -18,7 +18,8 @@ with col2:
     lon_col = st.text_input('Enter the longitude column name', 'Long')
 
 uploaded_file = st.file_uploader("Choose a CSV file")
-if uploaded_file is not None:
+
+if st.button('Process CSV') and uploaded_file is not None:
    df = pd.read_csv(uploaded_file)
 
    with st.spinner('Fetching land coverage data...'):