Problems remain with codedFixedValueFvPatchScalarField:

- readIfModified() notices change on system/codeDict, but the
  codeProperties::setUnmodified() means that only a single entry will
  get processed

- it appears that while dlclose() may (or may not) be actually closing
  the library, there are probably still references about. This means
  that a subsequent reloading still points to the original functions
  and the lookup is not updated correctly.

- use newly introduced makeRemovablePatchTypeField to avoid this problem

- file changes were not getting noticed, made modified 'sticky'

- the resulting SHA1 can be written and parsed directly
  without any ambiguities when it starts with a digit
  eg, SHA1: _024ea2fa570968069f52f55aa47ee82ac417f5f6

- provide same optional prefix functionality in conversion to string
  and in comparison with strings

- add SHA1Digest::null