added entropy as optional field type

[bobmyhill]

This PR adds entropy as a new field type.

• I have read the guidelines in our CONTRIBUTING.md document.
• I have followed the instructions for indenting my code.
• I have tested my new feature locally to ensure it is correct.

[MFraters]

You should also add it to

aspect/source/simulator/parameters.cc

Lines 1217 to 1229 in c6a5fc5

	prm.declare_entry ("Types of fields", "unspecified",
	Patterns::List (Patterns::Selection("chemical composition|stress|strain|grain size|porosity|density|generic|unspecified")),
	"A type for each of the compositional fields requested. "
	"Each entry of the list must be "
	"one of several recognized types: chemical composition, "
	"stress, strain, grain size, porosity, general and unspecified. "
	"The generic type is intended to be a placeholder type "
	"that has no effect on the running of any material model, "
	"while the unspecified type is intended to tell ASPECT "
	"that the user has not explicitly indicated the type of "
	"field (facilitating parameter file checking). "
	"If a plugin such as a material model uses these types, "
	"the choice of type will affect how that module functions.");

. Otherwise looks good to me.