Pass SIRIUS_CONFIG environment

[toxa81]

Read SIRIUS_CONFIG from environment and initialize context from the provided file.

Use case: do not pass -sirius_cfg to qe-sirius, use environment variable instead to pass the file name.

Definition of done: passing SIRIUS_CONFIG as environment variable works; tested on QE

[mtaillefumier]

Do you have any use case why this functionality is better than passing a file in the command line.

[toxa81]

Exciting code. There adding new command line is discouraged.