Some slight speedups of convert units

[Carter12s]

While running the "help_functions_chemdose_ph" vignette I started profiling some of the code as I noticed it was taking a while to run on my computer. Specifically I was profiling this snippet with profvis

profvis({
          solve_column <- raw_water %>%
                  chemdose_ph_chain(input_water = "balanced_water") %>%
                 cross_join(raise_ph) %>%
                  solvedose_ph_once(input_water = "dosed_chem_water") %>%
                 select(-c(defined_water:dosed_chem_water))   
          })

The baseline case on main was taking ~7.5 seconds to run on my machine:

With convert_units accounting for ~2.1 seconds of runtime. Although results are very messy.

After these optimizations the example is down to ~7.0 seconds, and convert_units is down to 1.9 seconds. Not huge gains, but a start.

Going to continue to optimize from here, but wanted to at least show these gains were possible.

[Carter12s]

Okay I've applied a memoization approach of pre-computing the standard unit conversions and then looking them up instead of recalculating each time.

This results in the same example taking 5.8s on my machine (~22% faster) and convert_units is now down to 800ms aggregate time instead of 2.1s (~61% faster).

I'm less familiar with how to distribute R data files, but it does look like storing a table of pre-computed unit conversions will be a significant speedup to the overall package.