Fix signal registration

[ericpre]

Now the signal should be registered and setting the signal_type will assign to the correct class when loading data or changing class (dimension change or from to lazy signal).

• Fix signal registration,
• remove unnecessary boiling plate code,
• add test.

To verify:

>>> import hyperspy.api as hs
>>> hs.print_known_signal_types()
+----------------------------+----------------------------------+----------------------------+----------+
|        signal_type         |             aliases              |         class name         | package  |
+----------------------------+----------------------------------+----------------------------+----------+
|           CL_SEM           |  CLSEM, cathodoluminescence SEM  |       CLSEMSpectrum        | lumispy  |
|          CL_STEM           | CLSTEM, cathodoluminescence STEM |       CLSTEMSpectrum       | lumispy  |
|             CL             |     CL, cathodoluminescence      |         CLSpectrum         | lumispy  |
|     DielectricFunction     |       dielectric function        |     DielectricFunction     | hyperspy |
|          EDS_SEM           |                                  |       EDSSEMSpectrum       | hyperspy |
|          EDS_TEM           |                                  |       EDSTEMSpectrum       | hyperspy |
|            EELS            |             TEM EELS             |        EELSSpectrum        | hyperspy |
|             EL             |     EL, electroluminescence      |         ELSpectrum         | lumispy  |
|          hologram          |                                  |       HologramImage        | hyperspy |
|        Luminescence        |       LuminescenceSpectrum       |        LumiSpectrum        | lumispy  |
|        Luminescence        |     TRLumi, TR luminescence      |       LumiTransient        | lumispy  |
|             PL             |      PL, photoluminescence       |         PLSpectrum         | lumispy  |
+----------------------------+----------------------------------+----------------------------+----------+