Calculation input context

[RedNeath]

Creating the structure to handle the use of variables (equivalent of the value of a cell in a spreadsheet) in the calculation.

NOTE: Those variables can be of the following types (may be extended in the future)

• Number (with or without decimals)
• Rate (to handle percentages)
• String (formats not supported, a formatted string will remain formatted)
• Date (as UNIX timestamp, basically a big number)

This structure will be a "data transfer" structure (not quite a DTO, as C is not object oriented, but the principle remains the same). That means there won't be any logic in the data from this structure, as it only aims at passing input data to the programme.

Its name should be formula_variable. A context is formed of n variables, which means it in fact is an array of formula_variables.

It should contain the following:

• An identification name (only upper-case alphanumerical characters supported, with a maximum length of 10 characters, not including the sentinel). In a spreadsheet, that would correspond to the cell coordinates (A1 for instance).
• A value, from one of the types cited above (most likely a pointer towards its value, so that we don't have to add a variable in the structure for each type).
• Information about its type, which shall be responsible (to a possible extent) for getting the actual value from the pointer.

[RedNeath]

Not up-to-date, see README.md

[RedNeath]

Base done. => Handling functions still to be done

[RedNeath]

See incoherences with the main readme (and update what needs to be updated, especially the code blocks presenting examples).

To check:

• type casting (used float instead of double for number & rating)
• edit readme, rate is not in percent. Some logic above could be doing the trick, but it makes no sense to add such a conversion in the engine directly.
• edit readme, as the context cannot really be made that easily => encourage users to use the functions that creates the variables for them (because they do not get confusing when it comes to memory ownership).
• in the computation of a formula block, the "variables" have to be changed for "context", for better clarity

The rest is mostly correct, if the code works

[RedNeath]

Almost done testing, some tests need a bit more work to really ensure the functions works as described in their documentation.