BUG: fix unit conversion for array with dtype int8

[neutrinoceros]

This dtype was apparently overlooked in PR #56

import unyt as u
a = u.unyt_array(1, "cm", dtype="int8")
a.to("km") 

fails with

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-fe344d50e9a4> in <module>
      1 import unyt as u
      2 a = u.unyt_array(1, "cm", dtype="int8")
----> 3 a.in_units("km")

~/dev/python/yt-project/unyt/unyt/array.py in in_units(self, units, equivalence, **kwargs)
    845                     )
    846             new_dtypekind = "c" if self.dtype.kind == "c" else "f"
--> 847             new_dtype = np.dtype(new_dtypekind + str(dsize))
    848             ret = np.asarray(self.ndview * conversion_factor, dtype=new_dtype)
    849             if offset:

TypeError: data type 'f1' not understood

the root cause is that float8 dtype doesn't exist. The fix is to force minimal itemsize in bytes to 2, rather than copying it directly from the input dtype, so input array with dtype int8 gets converted to float16.

While I'm at it I also patched unyt_array.convert_to_units so that it raises a clearer error message in case the user attempts to convert a int8 array in place (which isn't possible, I think)

[neutrinoceros]

@ngoldbaum does this look okay to you ?