fix scenario suffix handling for case sensitive file systems

[sfo]

On case-sensitive file systems, loading a scenario file with upper case .SCN extension (like provided by bluesky itself) fails with a file not found exception since the method for reading the file replaces the suffix with lower case .scn.

[jooste]

This might not be necessary if we go for a complete solution for case sensitive file systems, see also my comment in the IC IC PR.

[sfo]

In the other comment you wrote "[...] adapt the scenario loader [...]". Do you refer to the readscn method?

I just checked my fix, which fails to load scenarios without a suffix (like in the IC IC case). For these, a .sim suffix gets appended implicitly, which can fail on case-sensitive file systems. Even worse, there might be multiple possible candidate files, for instance a .SCN and a .ScN and a .sCn (even though, this is unrealistic, it could happen).

How should this be handled? Maybe just support .SCN and .scn and only fail if both or neither variant could be found on loading?