Implement daily penman function

[sbegueria]

Función para Penman-Monteith con datos diarios

[rdstern]

@sbegueria I follow this, which I interpret as a question, or a plan for the future.
Reading your documentation and looking at the penman function as you describe it, I assume it can now be used on other than monthly data. I also note from #62 that some users are trying Penman with daily data.

I write to check that there should now be no problem, that you can see, using penman with daily data? I ask, partly because, if so, then this is something I would like to encourage users to consider. Namely when they have daily data, (and want to use SPEI on a monthly basis, they could either:
a) Compute penman with the daily data and then average, or total, the penman values. Or:
b) Average the inputs to penman onto a monthly basis and then calculate the montly penman values.

I don't think the results will be very different, but (as I understand it) the physics behind penman are built more for the daily calculation, and hence - if true - then that could be encouraged.

I'd welcome your reaction.