Pass spec pythonpath directly to spectre

[guilara]

Proposed changes

Since the spec exporter might be a bit difficult to set up, I would like to use the spec python scripts independently of it (still useful for using the pipeline). Could we pass the path directly if it fails to find spec like this?

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Further comments

[nilsvu]

I suggest you just move up the line that sets the pythonpath so it's always set, like this:

find_package(SpEC)

# Make SpEC scripts available in Python. These can be used until we have ported
# them to SpECTRE.
if (SPEC_ROOT)
  set(PYTHONPATH "${SPEC_ROOT}/Support/Python:${PYTHONPATH}")
endif()

# That's it. No need to do anything else in this file.

[guilara]

Oh, right. That's simpler. Done.