Make sure that innerArea is not negative. Move sending out of messages
to the bottom of the process function. Add sending out of
concentration to the buffer and diffusion to other difshells in
DifShell and diffusion to other buffers in DifBuffer in reinit. This
matches genesis implementation.

In DifShell reinit set C (calcium) to Ceq. In DifBuffer reinit
calculate bFree (free buffer) and bBound (bound buffer) to steady
state value. This matches genesis implementation.

Integration results were incorrect, because the helper variables were
not reset after every iteration.

Most of Cinfo code is moved to DifShellBase, so it got deleted from
DifShell. Method's names are changed, because they are now inherited
from DifShellBase.

Base classes are necessary for implementing hsolve.

Reintroduced implementation of Difbuffer used moose objects that have
been since rewritten and renamed.  Change used moose objects to
current version.

Change process_0 to proc, to match other classes which need to be
driven by ticks.

π is defined as a constant in the headers, no need to create a
variable. Also constify some other variables. No functional change.

New getters and setters are added.

Fixed grammar and typo.

Added links to install/index.rst file; and also to online file link.

Fixed typos.

removed url. URL is listed on the project page.

Added clang compiler also to travis.