Build the whole de Rham complex in the domain

[tommbendall]

This PR:

• expands the space building so that we always build the H1 space and HCurl space (if this exists), thus giving us the whole de Rham complex
• specifies what the appropriate H1 space is for a particular de Rham complex, which should be used for any vorticity field, thus addressing CG space used for vorticity field #338
• adds the correct handling for the BDFM space, thus addressing Implement BDFM family #254
• there is a unit-test for space creation
• introduces space_names as an argument and property for equations sets. This means that the space for prognostic variables can now be specified by the user (although the defaults are of course the same). This addresses It is assumed that the compatible finite element spaces have the same ordering as the prognostic variables #342
• the space that was registered as "DG" is also registered "L2"
• I changed some " to ' in equations.py which just makes the code a bit more consistent

[jshipton]

This looks fantastic - thanks @tommbendall !!