No major functional changes, just code polishing for smells and performance

[adamhsparks]

The biggest change is that rain.at.TS in SPIchanges() must now be a matrix generated by TSaggreg() only. This helps ensure the data quality further.

• I've removed {dplyr} but introduced {rlang} as an Imports,
  • Some errors are more informative now thanks to rlang::arg_match() (https://github.com/adamhsparks/SPIChanges/blob/b1c054ef94925d4b61a8061a426f8b672f6a877c/R/SPIChanges.R#L88),
• I've removed namespacing (i.e., pkgname::fnname() and relied on @importFrom only,
• I've reduced repetition where possible, e.g., creating functions and using lapply() to call them rather than repeating the same code over multiple lines (https://github.com/adamhsparks/SPIChanges/blob/b1c054ef94925d4b61a8061a426f8b672f6a877c/R/SPIChanges.R#L166, https://github.com/adamhsparks/SPIChanges/blob/b1c054ef94925d4b61a8061a426f8b672f6a877c/R/SPIChanges.R#L539),
• Spellchecked the package and updated the WORDLIST,
• Added further details to documentation where possible to help users understand the package,
• Spellchecked and performed some formatting in the README.

This is really nice work, @gabrielblain! Thank you for inviting me to be a part of it.

[gabrielblain]

Thanks @adamhsparks

[adamhsparks]

I'm working my way through the paper now that I've familiarised myself with the work. Working through this package definitely helped me understand the paper. This is very neat work!