Added PoC for npk modification as asked in #3

tools/demo_modify_nkp_packages/poc_modify_npk.py

@@ -0,0 +1,56 @@
+from pathlib import Path
+from pprint import pprint
+
+from npkpy.npk.npk import Npk
+
+
+## KEEP IN MIND: MODIFICATIONS WILL INVALIDATE THE NPK PACKAGE SIGNATURE!
+##               THE ROUTER WON'T INSTALL THE PACKAGE.
+
+
+def modify_poc():
+    npk_file = Npk(Path("tests/testData/6_48_4/gps-6.48.4.npk"))
+
+    # print overview
+    print("----Overview--------------------")
+    pprint([f"pos: {pos:2} - Name: {cnt.cnt_id_name} (id:{cnt.cnt_id:2})" for pos, cnt in npk_file.pck_enumerate_cnt])
+    print("-------------------------------")
+
+    # The following code example will modify the payload section of PckDescription
+    CNT_ID = 4  # PckDescription
+
+    print("Payload original:")
+    print_overview(npk_file, cnt_id=CNT_ID)
+
+    print("overwrite payload - same size:")
+    npk_file.pck_cnt_list[CNT_ID].cnt_payload = b"a" * 25
+    print_overview(npk_file, cnt_id=CNT_ID)
+
+    # Modifying the size of the payload can affect the whole npk package and
+    # forces recalculations in other containers of this package
+    print("Payload new - small size:")
+    npk_file.pck_cnt_list[CNT_ID].cnt_payload = b"b" * 10
+    print_overview(npk_file, cnt_id=CNT_ID)
+
+    print("Payload new - increased:")
+    npk_file.pck_cnt_list[CNT_ID].cnt_payload = b"c" * 100
+    print_overview(npk_file, cnt_id=CNT_ID)
+
+    print("Write File: modified.npk")
+    Path("modified.npk").write_bytes(npk_file.pck_full_binary)
+
+    # Parse the new npk file as shown blow:
+    # $ npkpy --files modified.npk --show-container
+
+
+def print_overview(npk_file, cnt_id):
+    cnt = npk_file.pck_cnt_list[cnt_id]
+    print("Cnt payload:      ", cnt.cnt_payload)
+    print("Cnt payload len:  ", cnt.cnt_payload_len)
+    print("Cnt len:          ", cnt.cnt_full_length)
+    print("pkg len:          ", npk_file.pck_payload_len)
+    print("-------------------------------")
+
+
+if __name__ == '__main__':
+    modify_poc()