The tumour clock

You will write functions for the growth of a tumour that grows from one cell by doubling.

The idea. After $d$ doublings there are $2^{d}$ cells. A billion cells weigh about a gram.

Your tasks.

1. Finish cells. It gives the number of cells after some doublings.
2. Finish doublings_for. It gives the smallest number of doublings after which there are at least target cells.
3. Finish years_for. It gives the years taken by some doublings, if one doubling takes days days. Round it to one decimal place. Use 365 days for a year.