No files -> should have helpful error

[lionfish0]

Currently if no files provided, alignment.firstpass() will break with a confusing message:

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Cell In[6], line 1
----> 1 alignment.firstpass()
      2 alignment.secondpass()
      3 alignment.addmissingobs()

File ~/Documents/Research/alignment/alignment/__init__.py:347, in Alignment.firstpass(self, method)
    346 def firstpass(self,method='BFGS'):
--> 347     self.cameras[0].loc = np.zeros(3)
    348     self.cameras[0].orientation = np.zeros(3)
    350     params = np.random.randn(6*(len(self.cameras)-1))

IndexError: list index out of range

this happens as no cameras have been added to the cameras list (as no images have been taken) -- the code above fixes camera 0 to the origin etc.