Support for titrating doses and dose change schedules

[wildy]

Similar to this issue I opened for RxDroid, but sadly, it's been out of development for a while.

In Calendula it is already possible to do this, by setting multiple schedules for the same drug for different periods of time, but it'll be probably more convenient if it had support to change dosing over time instead of this 'hack'.

I'm proposing to add custom dosage rules for drugs that need to be titrated slowly.

We will also need a custom dosage plan widget that would allow to create a schedule for varying dosage with time. There should be a 'start date' field as well as the already existing 'end date' field as well.

Also, there are certain cases when you can't split pills, some drugs require to be taken as whole pills only. This is where this scenario becomes very useful.

EDIT: Also, it seems like some kind of increment/decrement control would be useful (say, (in|de)crease by n pills in m days).

Example of usage:

Suppose I have only two different entries for the same drug in Calendula, one for 25mg pills and another for 100mg ones. The dosage plan would be as follows:

1. (25 mg entry start date comes, it becomes active in Calendula)
2. 1x25mg for the first week
3. Then 2x25 mg for the next week.
4. Then 3x25 mg for the third week.
5. (100mg entry start date comes, the drug entry becomes active).
6. 1x100 mg for the next week.
7. 1x100 + 1x25 mg for the next week.
8. 1x100 + 2x25 mg for the next week.
9. 1x100 + 3x25 mg for the next week
10. (25 mg entry end date comes since it's not needed anymore and the dosage is stable at 200 mg now)
11. 2x200 mg until forever, or until the 100mg entry end date comes.
12. (100 mg entry end date comes, 100 mg entry becomes inactive, there are no active entries in Calendula anymore).

[guppyfaced]

This would a super addition.