Eliminating few compile time warnings

[shleym2000]

Fixes compiling problem

[shleym2000]

Without this change, there are multiple warnings like the following

1>C:\OpenNN\eigen\unsupported\Eigen\CXX11\src\Tensor\Tensor.h(76,49): warning C4554: '&': check operator precedence for possible error; use parentheses to clarify precedence
1>(compiling source file '../SandBox/OpenNN/opennn/adaptive_moment_estimation.cpp')
1>    C:\OpenNN\eigen\unsupported\Eigen\CXX11\src\Tensor\Tensor.h(76,49):
1>    the template instantiation context (the oldest one first) is
1>        C:\OpenNN\opennn\histogram.h(74,19):
1>        see reference to class template instantiation 'Eigen::Tensor<opennn::type,1,0,Eigen::DenseIndex>' being compiled