[dynoplibs] Add a coth() op
[pmc] Added a coth() pmc method
[t] coth todo tests now pass
[imcc] Added coth to IMCC_subst_constants

Not 100% sure if this is required, but it seems like it is a good idea that
imcc knows about coth.
[dynoplibs] Added acot() op
[pmc] Added acot() pmc method
[t] Corrected acot() tests in trans-infan.t

The previously commented-out tests were not testing for the correct values.
The test results used here have been checked against expected values from
Mathematica.
[imcc] Added 'acot' to IMCC_subst_constants
[doc] Updated ChangeLog after coth() and acot() additions
[pmc] Documented the acot() and coth() methods
Merge pull request #965 from paultcochrane/ptc/add_coth_acot

Implement coth() and acot() math functions