Perform some geometric RV computations in log space to increase
accuracy.

Add tests for logcdf, logsf and ppf geom RV functions.
MAINT: replace internal calls to `norm.pdf` with _norm_pdf
to avoid the public function call overhead. Closes gh-1710
Fixing Issue #2720

Fixing wrong docstring of dct function (Type III, norm=ortho)
https://github.com/scipy/scipy/issues/2720
Merge pull request #2819 from jwkvam/geom_log_space

Perform some geometric RV computations in log space to increase accuracy.
Merge pull request #2826 from ClemensFMN/patch-1

Fixing Issue #2720
Merge pull request #2822 from EvgeniBurovski/norm.pdf

replace internal calls to `norm.pdf` with _norm_pdf