Allow reading of parameters from input file.

Instead of specifying the parameters in the driver.py file, they are now read from a text file (see parameters_template.in for an example) if the name of this file is given as a command line parameter. Otherwise the defaults set in driver.py are used. The parameter parsing does some error checking (such as verifying that the type in the file matches that given by the default parameters).
Added functionality for printing out a warning message
Bugfix: Added missing modules to pressure solver __init__.py, mpi_utils was not imported correctly in parameters.py
Merge branch 'read_parameters_from_file'