rewrite ensure_cpg_file function to prevent frequent file writing

[yiqiaowang-arch]

Describe the PR
The original ensure_cpg_file function was called very frequently whenever get_zone_geometry is used. Particularly, when using the energy potential - photovoltaic-thermal panels functionality. In each separate PVT calculation, this function was called around 12 times.

The original implementation of ensure_cpg_file is to just write zone.cpg file to desired content: "ISO-8859-1". When using a cloud drive, for example, OneDrive, to sync the scenarios, such frequent writing will cause OneDrive to sync this .cpg file constantly. Sometimes this cloud drive sync will block CEA to write it, causing PermissionError when trying to write this file again, forcing CEA to exit the current batch simulation. For example, my batch of PVT simulation stops at 1728/1752, and it's hard to find out which one has not been simulated yet...

A straightforward solution will be first open the file with read access, check if the content is already there. If so, there's no need to change anything and just end the function; if not, then the file is re-opened using with, and this time the correct content is written into the file.

To Test
I have tested it on my own computer and make sure that this function worked correctly with both correct and wrong content in zone.cpg after several runs. However, there's not yet a systematic testing on all cases.

Steps to test this PR:

1. open zone.cpg in /inputs/geometry folder;
2. delete contents inside the .cpg file, save and close;
3. do a demand analysis for a building, or any other functionality that calls get_zone_geometry;
4. After the calculation, open the zone.cpg file again, confirm that the content is ISO-8859-1.
5. This test confirms that the goal of this function, which is still to ensure the encoding, is still working.

Expected behaviour
Behaviour should not be affected.

Extra Data Needed
None.