Initial new code for the partial differentiation of keys. The adapted
code for the series class is not there yet.
Change the monomial classes so that they always return a compatible key under differentiation.

Previously, a partial() wrt to a variable not present in the monomial or whose exponent/multiplier
is zero would return an empty key. Now the returned key is always compatible with the input arguments.
series: some work on the new partial method, with the tests split off into a separate file
as a first step to curbing the compile time of the series test.
sphinx: doc updates.
power_series: a couple of test additions.
sphinx: minor doc update.
pyranha: update the sin/cos wrappers to return the correct type, instead
of forcing it.
The big partial() change.

* finish re-implementing the generic series::partial() method,
* erase all term-specific differentiation,
* rework pbracket and canonical transformation check for the new generic method,
  plus with better function signatures using the new enabler style,
* adapt the custom derivatives logic,
* testing bits,
* adapt pyranha for these changes.
A detail header was not included in the CMake project.
sphinx: doc updates.