Remove use of `popen` for gzip file reading and writing

[gadfort]

Description

I happened upon the spef writing code and noticed it uses a popen to write the SPEF file (also for reading).

https://github.com/The-OpenROAD-Project/OpenROAD/blob/6e29df0746191b48963a35bf45422d76e2c9922c/src/rcx/src/extSpef.cpp#L1216 https://github.com/The-OpenROAD-Project/OpenROAD/blob/6e29df0746191b48963a35bf45422d76e2c9922c/src/rcx/src/parse.cpp#L140 https://github.com/The-OpenROAD-Project/OpenROAD/blob/6e29df0746191b48963a35bf45422d76e2c9922c/src/rcx/src/parse.cpp#L156

Suggested Solution

Don't call external programs to handle IO

Additional Context

No response

[maliberty]

@gadfort are you planning to tackle this one too?

[gadfort]

@maliberty I can take a look, but it's not code I've spent much time actually looking at (the writing seems straight forward-ish).

[maliberty]

Thanks - nobody has spent too much time with it...

[gadfort]

@maliberty I take back what I said about straightforward... I'll see what can be done

[maliberty]

Little in RCX is straightforward