new function that checks/creates a python environment.

[MadniAbdulWahab]

Fixes #617 .

Changes proposed in this pull request:

• A new function that (a) checks whether a python environment exists, and (b) if not, creates one from the specification of a path to a python installation.

[dominikbach]

Two suggestions:

• name the function pspm_check_python
• throw a warning if no python environment could be created, and return a status argument sts with values 0 or 1.