Support for packages wich are not (yet) needed by dev branch (FFTW3, mxpfit, Eigen3)

Several users need external packages in order to further develop mpdroot. If their newly created code will be incorporated into the dev (releases) of mpdroot, these packages will be made a mandatory dependency of mpdroot. In order to ease this transition we will add so-called dev packages that will be automatically loaded when present but not (yet) mandatory for other users. To use these external dependencies, one has to manually load them and they will become available inside mpdroot. For example, calling module add mpddev Eigen3 will make Eigen3 package available inside mpdroot. For the list of supported external packages, have a look at https://mpdroot.jinr.ru.