Fixed GPL license at top of files.

The GPL license at the top of every file was mangled slightly by
clang-format. This fixes the problem and adjusts how the text is
formatted to prevent it from happening next time clang-format is run.
Changed floating point equality comparisons.

Many places in the code were doing floating point comparisons with "=="
and "!=", which obviously can be problematic since floating point math
is not exact. Many of these were changed to use the "distinguishable"
function in util.c.
More floating point equality comparison changes.
Removed/commented dead code.

Code that was reported unreachable by clang was removed or commented
out. There were many sections with return/break/free statements which
couldn't be reached because of calls to mcell_error or some similar
function, which killed MCell before it could ever get to it. In a few
cases, some of the cleanup code was moved ahead of the fatal errors, so
that it was actually being called before the program quit. Simply
removing the code is likely not the best long term solution, as we might
want to be able to recover from some of these failures if they are used
in API calls. However, for the time being, there doesn't seem to be any
reason to leave in this dead code, since it's pointless and misleading
at best.
Minor refactoring of init_reactions.
Fixed clang warnings about switch statements.

Added missing cases for switch statements.
Fixed more clang warnings.

- Set uninitialized values
- Removed unused macros
- Fixed bad function casts
Merge branch 'master' of https://github.com/mcellteam/mcell