Fixed bug in tuning_curve where self.x_values would not be created

Bug was introduced when the _reduce_ticks method was changed from
placing ticks between two fixed values (0 to pi), to placing them
between the first and the last self.x_values. Fix involved creating
self.x_values even for non-cyclic tuning curves.
Added measure_center_contrast measurement class and GUI entry

measure_center_contrast measures the response of a central unit
as a function of the contrast of the central grating in presence
of a iso-oriented surround.