Minimum size requirements for the subdomains

[semi-h]

We need to investigate the minimum subdomain size that is safe in terms of accuracy. It is strongly linked to the alpha parameter in tdsops operators the size of the tridiagonal systems in the subdomain. I think it would be best to analyse this in tdsops class, each time a new instance is created. Two parameters that impact accuracy, n_tds and alpha is known at this level and we can simply warn the user in some cases or even terminate the program.

Originally posted by @Nanoseb in #132 (comment)