fmudesign: Detect merged cells in design matrix

[tralsos]

If users have merged Excel cells in their input to the design matrix, this can lead to hard-to-understand error messages.
It is possible to detect merged cells in an Excel sheet and warn the user that this is not recommended, and unexpected behaviour should be expected.

[berland]

See for example:

https://github.com/equinor/pyscal/blob/bce9b75aef23f8b03e9fdf60158eb4790cd725fc/pyscal/factory.py#L514

[tralsos]

When reading the design_input into a pandas dataframe empty cells and merged cells are both read as NaN, There will always be empty cells, so not sure how to separate those from merged cells.

[berland]

Let's put on hold until we find a solution. Probably not doable with Pandas directly, possibly with coding the excel -> dataframe conversion manually with openpyxl, which is probably some work.