implement delta calculation function

[sebkopf]

for example something modelled after https://github.com/isoverse/isoprocessor/blob/812646ebab0faff38c30ace78be82a16704aed66/R/calculations.R#L221 except it can be for all ratios at once given the base peak structure

aim for v. 1.4? ideally for flow injections AND dual inlet samples

important decisions to make:

• how to deal with segmented blocks' averaging and error calculation when bracketing sample/standard?

note issue #46 is also relevant for this as more automated sequence information is very useful to include

[sebkopf]

also consider a plot_block_data() function or something like it that makes it easy to visualize ratios after summary calculations (block might no be the best for flow injection analyses)