Add clear portfolio reset functionality in app.py

- Introduced a button to clear the portfolio dataframe from session state, allowing users to reset their portfolio easily.
- This enhancement improves user experience by providing a straightforward way to manage portfolio data during uploads.

app.py

@@ -179,6 +179,8 @@ with tab1:
     
     with portfolio_upload:
         st.info("If you have a portfolio of lineups, you can upload them here to see how they would have performed against the field")
+        if st.button('Clear portfolio', key='reset2'):
+            st.session_state['portfolio_df'] = None
         st.session_state['portfolio_file'] = st.file_uploader("Upload Portfolio File (CSV or Excel)", type=['csv', 'xlsx', 'xls'])
         try:
             st.session_state['portfolio_df'] = pd.read_csv(st.session_state['portfolio_file'])