implementing SPI

[jmigueldelgado]

User Behzad Navidi asked if implementing SPI would require significant additional changes to the code.

Answer:

Computing the SPI will only require a small
change in the code. The water balance D=P-E will no longer be necessary
and can be replaced with D=P. This would mean that the standardization
of the variable later could be made with the gamma function, but I
don't see a reason why the currently implemented log-logistic won't work.

That would require creating a new function spi.m that would be similar
to spei.m only with the small change described above. Definitely worth
doing.