Made AdiabaticConditions a plugin architecture as well.

This resulted in changes to some material models since we no
longer can ask whether get_adiabatic_conditions returns a null
pointer (it will never do now), but instead we need to ask whether
the adiabatic conditions are already calculated, which is a function
that every adiabatic conditions plugin must implement.
Leave a comment.
Use correct ifdef name.
Documentation updates.
Allow initial conditions plugins to override the initialize() function.
Let InitialProfile create a constant AdiabaticProfile.
Since we have a valid default value, we do not need to try-catch.
Activate the update function for AdiabaticConditions.
Updated documentation and marked functions as virtual.
Updated plugin description text.
Create the output directory if it doesn't exist.
Fix up a parameter. Take into account that a user may want to specify a nested directory for which we would need to create multiple levels.
Clarified plugin description test.
Only create the directory on processor 0.
Write warning message to std::cout instead of std::cerr.
Merge pull request #87 from gassmoeller/flexible_adiabatic_conditions

Flexible adiabatic conditions
automatic version number stuff

This adds a script increment_version.sh that does the required shuffling
of changes.h and the other .h for specific versions. The layout of the
files changed a bit so that automation is easy.
version info scripts

Introduced a ./VERSION file, output version when you start ASPECT,
scripts to update version info (this updates the two places above and
the manual).
update task list
Merge pull request #91 from tjhei/version_stuff

Automatic versioning, change layout of changes.h files