diff2Hours converts a difftime object into its numeric
value with units = 'hours'.
char2diff converts a character description into a
difftime object, following the code of
seq.POSIXt.
sample2Hours calculates the sampling time in hours described by a character
or a difftime.
P2E (power to energy) sums a series of power values (for
example, irradiance) to obtain energy aggregation (for example,
irradiation) using sample2Hours for the units conversion.