fix profiling/LotkaVolterra.vpz model

The name of the package for the dynamic used was wrong.
all: fix string comparison in CMakeLists.txt

Keyword EQUAL was used to check the CMAKE_BUILD_TYPE value and that did not
work. It is now replaced by STREQUAL.
differential-equation: add output_period parameter

Output of variables was performed at each time step of integration.
To reduce the number of outputs, the parameter output_period (default to 1)
allows to perform outputs every output_period integration steps.
diff-equ: remove PDF from source tree
diff-equ: add vle.output in build depends