Export needs to be able to handle export of registers with no fields defined.

As a user I want registers without any fields defined to be exported in a valid way for the target language/format.

At the moment an Exception is thrown:

Traceback (most recent call last):
  File ".venv/bin/csr-export", line 4, in <module>
    main()
  File ".venv/lib/python3.6/site-packages/csrParse/csrExport.py", line 54, in main
    packageOptions.exporterOptions.exporter.to_source( packageOptions, csrData )
  File ".venv/lib/python3.6/site-packages/csrParse/csr/exporter/c/export.py", line 110, in to_source
    thisMapRegister.addField( thisField.name, (thisField.lsb, thisField.msb) )
  File ".venv/lib/python3.6/site-packages/registerMap/elements/register/instance.py", line 187, in addField
    isGlobal = isGlobal )
  File ".venv/lib/python3.6/site-packages/registerMap/elements/register/register.py", line 293, in addField
    field = linkToExistingLocalField( existingFieldsThisRegister[ 0 ] )
  File ".venv/lib/python3.6/site-packages/registerMap/elements/register/register.py", line 276, in linkToExistingLocalField
    revisedField = self.__createFieldToRegisterMapping( localField, registerBitInterval, fieldBitInterval )
  File ".venv/lib/python3.6/site-packages/registerMap/elements/register/register.py", line 323, in __createFieldToRegisterMapping
    assert fieldBitInterval is not None
AssertionError