fix code for 2.5 compatibility
correction to TRmorrie
add TR14, TR15, TR16, as_trig

TR14 converts factored identity expressions to simpler forms

TR15 and TR16 are similar to TR5 and TR6 except that they work with
  inverse powers of sin and cos, like 1/cos(x)**2

as_trig converts an expression's hyperbolics to trig functions using
Osborne's rule after masking off pre-existing trig functions.
TR13 returns expression with all tan or cot
update for rules->stategies; add tests
use expanded form of TR13
wip - futrig added
add TR12i to fu.py

PR 1737
add preprocessing to TR2i
add preprocessing to TR8
remove TR0 and introduce TRmorrie before TR8
enable
2i 12i mods
fu definition mods
TR2i allows optional tan(x/2) generation
correct trig_split: factoring does not remove -1 from -x-y
add TR17 and modify futrig
TR12i modifications: factoring and correction
further futrig tweaks; method=fu method=futrig supported
enable extraction of -1 in _keep_coeff