`interp1()` errors due to undefined `start` variable

[btalb]

Error is throwing when using interp1():

In [12]: a = SE3(1, 2, 0) * SE3.Rz(45, unit='deg')

In [13]: b = a.interp1(0.5)
---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-13-efa36fbc2384> in <module>
----> 1 b = a.interp1(0.5)

~/.python-venv/lib/python3.8/site-packages/spatialmath/baseposematrix.py in interp1(self, s)
    477         s = np.clip(s, 0, 1)
    478 
--> 479         if start is not None:
    480             assert len(start) == 1, 'len(start) must == 1'
    481             start = start.A

UnboundLocalError: local variable 'start' referenced before assignment

Error occurs for both SE2 and SE3 classes.

A workaround is to call interp() explicity with the identity pose, e.g.:

SE3().interp(a, 0.5)

[petercorke]

This is a poor cut and paste from interp() plus I went back/forth a lot with trinterp. Fix pushed to GH just now.