Python interface (#262)

* Added first working python interface

* Tidied up python interface

* Further simplification of python

* Added example which should be deleted later

* Added functions for getting values from plumed to python

* Added regression test for python interface

* Added mechanism to return exceptions from plumed to python

* First commit of experimental stuff to catch exceptions in python correctly

* Got rid of unecessarily duplicated method in PlumedMain

* Fixed crazy MAC input encoding

* Oops, had also broken the script..

* Removed debug messages. Works on Ubuntu Linux

Linus rules. Apple sucks.

* Fixed PlumedMain passing of data from plumed to outside code

* Hard coded real precision for python interface - this should have been with the last commit

* Added first go at working makefile for python interface

* Added options to configure to build python interface from makefile

* Small changes to where loading of python interface is done

* Removed unecessary wrapper c++ function from python interface

* Made python install work in a way that is consistent with the rest of plumed

* Added pythonpath to travis.yml

* Added installation of Cython to travis.yml

* Added ifdef block to stop cppcheck throwing an error

* Using uniqu_ptr for DataFetching object in PlumedMain

* Added install command for numpy to travis

* Added string to tell me whether plumed+python is being built

* Small change in travis.yml file to see if I can get python compile working

* Compilers for cython are now specified from Makefile.conf

* Now printing stuff on environment to try to work out why linking doesn't work

* A new attempt to fix the travis-ci issues

* Got rid of troubling TRAVIS_PYTHON_FLAG

* Small change to see if we can get this to work on travis

* Now setting LDSHARED from LDSO in makefile

* Added something to print everything in the python path when we run the python test

* Fixed python path in travis.yml

* Removed stuff for debugging configuration on travis

* Customized conda install (for python) in travis.yml so that osx version is installed for osx and linux version is installed for linux

* Set TMPDIR in .travis.yml to hopefully get osx to compile PLUMED + Python

* Made it so that python interface is rebuilt when user does make install so that correct paths are used

* Made it so that python interface is rebuilt in staging area during install procedure

* Corrected regtest script so that python tests are dealt with the same as everything else