Fix issues/2331: Bode plot returns continuous phase
BUG: issues/2331, change to assert_almost_equal and style improvement
ensure analyticity of associate Legendre function of first kind
removed superfluous comments
adapted documentation
removed known failure for clpmn test
improved documentation of lpmn and clpmn
BUG: special: fix square root conventions in clpmn and lpmn

Having the branch cut in (-1, 1) requires some care in the square roots.
TST: special: fix mpmath tests for legenp
DOC: special: adjust lpmn/clpmn documentation
do not allow evaluation of clpmn on the cut [-1,1]
remove exception if argument for clpmn is on the cut
BUG: special: return inf rather than 1e300 in clpmn/lpmn to signal infinity
Merge pull request #2800 from gertingold/gh-2396

BUG: special: ensure analyticity of associated Legendre function of first kind

gh-2396 discusses the problem of phase factors in the Legendre function
for complex arguments. In fact, the Legendre function of the first kind
as it is implemented, exhibits phase jumps on the unit circle and thus
is not analytic.

These commits introduce a new function clpmn which provides an analytic
associated Legendre function of the first kind. The existing function
lpmn in the new version raises an exception if used for complex
arguments. The present implementation follows the conventions stated in
http://dlmf.nist.gov/14.21 with a cut on the interval (-1,1). Some tests
have been added to test_basic.py focusing on the behavior close to the
real axis and close to the unit circle. The latter tests the absence of
phase jumps which were present before. Also, one test in test_mpmath.py
has been adapted to the new function.
DOC: release: document the change in lpmn
Merge pull request #2791 from andbo/master

BUG: signal: Bode plot should return continuous phase