Added a switch so that one can override the defaults of the impi
installation. Defaults are to wrap the system compilers with mpicc,
mpicxx, mpif90, ... The switch overrides this behaviour by setting some
environment varaibles. These environment varaibles also affect the
behaviour of mpigcc and mpigxx so I've added aliases to give back the
expected behaviour that they wrap the GNU compilers.
Update impi.py

compiler -> compilers
Update impi.py
Update impi.py
Update impi.py

Updated based on comments
Update impi.py
Update impi.py
Update impi.py
fix typos
Merge pull request #1 from boegel/change_impi_default_compiler

fix typos
check compiler env vars before using them
Merge pull request #2 from boegel/change_impi_default_compiler

check compiler env vars before using them
Update mpiicc etc. for consistency
Update impi.py

Added individual triggers for the environment variables and the aliases, as well as a catch-all for everything
Update impi.py

Added a debug error statement
minor style fixes
Merge pull request #3 from boegel/change_impi_default_compiler

minor style fixes
Update impi.py

Typos
last cleanup
Merge pull request #4 from boegel/change_impi_default_compiler

last cleanup