Adding Transition Tempering to METAD (#233)

* Added on-the-fly tracking of a METAD action's maximum bias.

* Added nearest grid neighbor functions to tools/Grid

* Added path barrier finding routine for gridded functions in tools/Grid.

* Added on the fly tracking of transition bias quantities in METAD.

* Added 1D and 2D METAD transition bias calculation regtests.

* Added transition tempering to METAD.

* Added regtest for transition tempering including TTBIASFACTOR, TTBIASTHRESHOLD, and TTALPHA.

* Added regtest for transition tempering including TTBIASFACTOR, TTBIASTHRESHOLD, and TTALPHA.

* Update MetaD.cpp

removed trailing character, line 398