integration-axis, x-array, y-array, <wrap_period>
Return the integral array of a data array via trapezium integration

This is the function to use to integrate irregularly sampled functions.
x-array is the array that will be integrated against, while the 1D
arrays along axis 'integration_axis' are the functions to be integrated.
   The function returns the array of integrals in a 3D array with the
first N-1 dimensions used, where N is the number of dimensions in the
original data array. In order to get just the dimensions used you will
have to use array indexing notation on the function return value. eg.

float ispec(100)
float spec(100,10)
ispec = trap_int(1, ramp(1,10), spec)(,1,1)

This integrates the 10 values along the second axis (axis=1) of spec at
each element of the 1st axis (axis=0) of spec, against a ramp of x values
1,2,3,...,10. It returns a float (100,1,1) 3D array of which we only
require the 1st dimension. This is extracted by the (,1,1) index specification.

Optionally if wrap_period is given then the integral is assumed to be of
a periodic function. The integral over one period of a periodic function
cancels out the periodic components of the function, leaving the
zero-offset of the function multiplied by the period. Thus if wrap_period is
given then the zero-offset itself will be returned instead of the
integral. In this case the data must not stretch over more than a period
so it may need to be folded first using fold(). It is unlikely that it
will cover exactly one period, so the final point in the integral,
placed at xmin+period will be the first point - hence the name
wrap_period.