TMP (uncompilable): Attempting to use basic data types from module data_kinds_4neuro_m + attempting to solve compilation problem

* Added missing 'return' statements to constructor in Doxygen comments
* 'sigmoid' neuron renamed to 'logistic'
* Logistic neurons' activation function changed to generalised logistic
function (added new constructor with 'alpha coefficient' parameter)
* Removed out-commented blocks of code with cancelled interconnection_t
dependencies

ENH: Implemented classes mock_neuron_t, binary_neuron_t, linear_neuron_t and sigmoidal_neuron_t in the module neuron_m.

FIX: Fixed constructors of interval_connection_t by adding 'intent(in)' to the input parameters (neuron_t pointers)

ENH: connection_test.f90 renamed to connection_mem_leak_test.f90 along with its binaries and rewritten to serve only as a binary file for Valgrind check. Its former use as unit test is now adopted by FUnit .fun files.

NEW + ENH: Added new module normal_m to generate random numbers from Gaussian distribution + simplified CMakeLists.txt