add check of ngrid that is a float
(#248)
updated comments on PowerSpectrum class to note possible use of TabulatedPk objects
(#305)
improved exception message
(#305)
Merge branch '#248' into #305
updated unit tests, demos for change in grid_nx vs. ngrid
(#305)
removed requirement that P(k) be defined at zero, just set it to zero there
(#305)
properly fix P(0)=0 whether P is a float or an array
(#305)
power function MUST return somethign the same size as it is given - check this explicitly
(#305)
added prelim version of splining in log k or P
(#305)