Test sparse function.
Test sparse functions in derivable-function.
Fix jacobian computation from gradients when sparse matrices are used.
Add experimental support for terminal colors (Linux-only).

When writing to std::cout or std::cerr, one can now use dedicated
stream manipulators to change the current font color.

Two API is provided:
- per-color API (red, green, ...)
- per-semantic API (fail, warn, ok)

When possible, please prefer semantics over color to ensure that the
user will have a consistent look'n feel.

This feature is currently only available on Linux.
Display functions with multi-line name properly.
Remove spurious inline keyword.
Fix DerivableFunction test.
Display multilines DifferentiableFunction/TwiceDifferentiableFunction properly.
Remove useless file.
Use sparse vector API when possible.
Fix Clang warning.
Fix warnings and add getter for numeric-linear-function.
Update to new test suite.
Add sparse support for numeric-linear-function.
Improve Gnuplot Jacobian display.
Implement cached Jacobian (when impl_jacobian is overriden).
Fix print for CachedFunction