implement custom idlpy location

[trittsv]

Previously the _idlpy could only be loaded from a location returned by python3 program.

This PR adds a command-line option -y <path> to use a custom _idlpy lib.

This is needed when you want to use idlc python without any pip package installation. For example CycloneDDS-Insight needs this because there idlc should use the binaries inside the App.

@eboasson can you have a look at this PR? 😊

[trittsv]

oh i missed that with -l its possible to pass a path ... sorry

[eboasson]

No worries 🙂